In an interview, Garry Kasparow stated that each generation of chess players knows more about the game than the previous one. He applies this thought concretely to the question of who is better - him or Magnus Carlsen. He seems to imply that Carlsen is the stronger player because of this.
What are the kinds of things that succeeding generations have learnt? Have there been well-known breakthroughs in understanding chess?
The knowledge of previous generations was summed up in Aron Nimzowitsch's book, published in 1929, called My System. The advances in chess knowledge up to Kasparov's time are best summed up in John Watson's 1998 book Secrets of Modern Chess Strategy: Advances since Nimzowitsch.
It is perhaps worth revisiting My System to remind ourselves of some of what was knowledge in 1929:
In between Nimzowitsch and Watson it's worth mentioning Ludek Pachman and his 3 volume Complete Chess Strategy first published in Czech in 1960. He deals extensively with the Minority Attack, which was around in Nimzowitsch's time (Capablanca used it) but didn't make it into My System.
Watson's main departure from Nimzowitsch centers around his principle of Rule Independence. What this boils down to is the idea that rules, a la Nimzowitsch, are for general guidance only and are not set in stone. Calculation is more important than rules. In this respect Watson captures the essence of modern chess in the age of computers. Perhaps also worth noting that Kasparov was the first of the computer generation pioneering, as he did, use of Chessbase.
To demonstrate the principle of rule independence Watson borrows an Andy Soltis example:
[fen ""]
[Title "Khlavin vs Zhdanov, Latvian Champs 1961"]
[Startply "20"]
1. e4 c6 2. Nc3 d5 3. Nf3 g6 4. d4 Bg7 5. h3 a6 6. Bf4 Nf6 7. e5 Ng8 8. Qd2 b5 9. Be2 h6 10. O-O-O e6 11. g4 Nd7 12. Bg3 Bf8 13. Rdf1 Nb6 14. Nd1 a5 15. Ne1 b4 16. Nd3 Nc4 17. Qe1 Qb6 18. b3 Qxd4 19. bxc4 Qa1+ 20. Kd2 dxc4 21. Nf4 Qa2 22. Ke3 Bb7 23. Qd2 g5 24. Nh5 c3 25. Qd3 Rd8 26. Qe4 Bc5+ 27. Kf3 Rd4 28. Qe3 Qd5 0-1
After Black's 10th move Soltis writes, sarcastically,
It doesn't take long to conclude that White has a very strong game. He has developed nearly all of his pieces while Black's only developed piece, his king's bishop, bites on granite. Black's queenside is full of holes on dark squares and he has just locked in his queen's bishop. A quick mating is assured, you might conclude. And you'd be right.
Dynamics change everything. Pieces gain their values not from a hard and fast rule but from the position and their possibilities which are ever changing. The Initiative has a value which can also change everything. Watson devotes a chapter to the Exchange Sacrifice.
This question needs a book. A rough sequence of changing understanding could be.
This is a woefully inadequate list. I could tell a whole story just about time controls and time management. To learn more about Garry's take I recommend his amazing series "My Great Predecessors."
I disagree with most (though not all) of the points mentioned in existing answers, and find them to arise from a rather naive and unsophisticated understanding of the game. I will first begin by explaining why some the points already mentioned are NOT the answer to the question.
One of the existing answers presents the answer as saying that prior to the modern era, people believed that chess was based on rules like controlling the center, whilst the new advances involve the "initiative" and "exchange sacrifice" and an understanding of "rule independence". This is totally ridiculous, because players all the way back to Paul Morphy, one of the earliest 'great' players of the game, played extraordinarily "romantic" chess which was all about sacrificing pawns for activity. Tal is also another famous, more recent example. To the contrary, modern players are more likely to play slow, maneuvering chess compared to the "romantic" era of Morphy and Andersson, like the countless Berlin defences (popularised by Kramnik in the 2000 World Championship against Kasparov) and Ruy Lopez and Giuoco Piano slow maneuvering chess seen nowadays at top level.
The books by Nimzowitch and Watson, and indeed any chess book in general, merely represent the points that the author thinks to be instructive for the reader and valuable to include in the book. The idea that this can be taken to be a summary of existing knowledge in chess, many aspects of which (e.g. a strong chess player's "intuition") is not even possible to really write down in a book the first place, is rather naive. All of the points mentioned in Nimzowitch's book was surely already understood to some depth by Paul Morphy, and even by any average 2000 rated player in 1900 or in the modern day. For example, the "exchange sacrifice" is an idea that even 1000 rated beginners have probably heard about, and these surface-level words and phrases have no relevance to the process of thinking at top-level chess. This has absolutely nothing to do with what Garry Kasparov was talking about when he mentions the advance in chess knowledge.
The real difference primarily lies in opening theory.
Prior to the age of engines, strong players who are looking to gain an advantage as white in the opening would need to discover these ideas for themselves through extensive analysis and great creativity. In complicated positions, it may often not even be clear whether or not a certain idea is good; evaluating these positions is a very difficult and sometimes tedious task (in terms of analysing lines). Nowadays, if Magnus Carlsen were trying to prepare for a World Championship match, he (or his team of seconds) could simply look to the engine for reliable analysis of lines and there is more or less no chance of making a severe error in analysis that could cost him the game by walking accidentally into a lost position out of the opening. Of course, there is still a lot of work to be done, in the sense that the opponent has access to engines too so it is difficult to come up with creative ideas to surprise the opponent, but it is still a great advantage in terms of quality of analysis compared to the pre-engine era.
Additionally, from the decades of games that have been played since Kasparov's time, chess theory in various popular openings have developed and improved, so that the top players know a lot more details about various lines than Kasparov would have been able to know. This is simply because more ideas would have been played by strong players, winning some nice games, and everybody (in the top-level chess world) takes notice and learns the lines too. Some topical examples includes detailed analysis in popular lines such as the Najdorf and Berlin, as previously mentioned. (Though I will admit that I am not strong enough to give detailed analysis of all of the intricacies of these lines and how they evolved over the past 20 years, and you should probably not believe anyone who claims to do so unless they are a strong GM.)
Another concrete example is that of openings like the King's Indian Attack, King's Indian Defence, and Queen's Indian Defence, all of which have been borderline refuted by engines. For example, in computer championship games, the side which plays the opening has an overwhelming disadvantage and loses a large proportion of the games. This is true even for openings like the Caro-Kann and the French defence. This has led to a drop in popularity at top level for many of these openings, when they had been popular even just a decade ago. For example, the famous game Gelfand-Nakamura from 2010 ("Nakamura's immortal") is a great example of a tremendously complicated middlegame where Nakamura won in the King's Indian Defence with the black pieces, where a critical position involving queen sacrifices on consecutive moves occurred.
[fen ""]
[Title "Gelfand vs Nakamura, World Team Championships 2010"]
[Startply "48"]
1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Nf3 O-O 6. Be2 e5 7. O-O Nc6 8. d5 Ne7 9. Nd2 Ne8 10. b4 f5 11. c5 Nf6 12. f3 f4 13. Nc4 g5 14. a4 Ng6 15. Ba3 Rf7 16. b5 dxc5 17. Bxc5 h5 18. a5 g4 19. b6 g3 20. Kh1 Bf8 21. d6 axb6 22. Bg1 Nh4 23. Re1 Nxg2 24. dxc7 Nxe1 25. Qxe1 g2+ 26. Kxg2 Rg7+ 27. Kh1 Bh3 28. Bf1 Qd3 29. Nxe5 Bxf1 30. Qxf1 Qxc3 31. Rc1 Qxe5 32. c8=Q Rxc8 33. Rxc8 Qe6 0-1
Despite this incredible win, which you might expect means that the popularity of the King's Indian Defence would surge amongst top level players, if you check the database, the last time Nakamura has played this opening as Black in a classical game was against Lenderman in the US Championships 2018 which ended in a draw. He has rarely played this even in rapid and blitz in serious tournaments since 2019. You will see similar trends if you look at the openings played in tournaments like the Candidates with a surge in popularity of reputable openings like the Ruy Lopez and fall in popularity of these imbalanced (and arguably engine-refuted) lines.
Hypermodern openings such as 1.b3 (Larsen's opening), 1.g3 or the Grunfeld offer an example that is less recent but maybe easier to understand. These openings were deemed to be dubious since they violated classical opening principles, opting to control the centre with long-ranged pieces (like a fianchettoed bishop) rather than pawns. The Grunfeld, for example, relies on active counterplay and undermining White's strong (but overextended) pawn center, where Black actively gives up center control. But once people such as Bent Larsen (a former 4-time World Championship Candidate) started playing moves like 1.b3, top players realised that hypermodern ideas were not as bad as once thought; indeed, these have now become popular openings in online blitz (with Nakamura, the currently FIDE blitz rated #1 in the world frequently playing 1.b3 in online blitz) and even in serious classical OTB tournaments (Maxime Vachier-Lagrave famously has the Grunfeld in his repertoire). To be clear, these openings have been known since more or less the 1920s, so Kasparov definitely wasn't talking about them when referring to new knowledge of modern players. Nevertheless, they offer an illustrative example of the nature of advance of chess knowledge that is easier to understand for non-GMs.
TL;DR What do modern chess players understand about the game that previous generations did not? The answer is detailed intricacies in many different lines of opening theory. This is why it is reasonable to think that Magnus Carlsen, or maybe even Fabiano Caruana, or Levon Aronian, or Ding Liren, or Ian Nepomniachtchi, at their peaks would likely (or at least have a real chance to) best Garry Kasparov at his peak. It is simply an unfair comparison because Kasparov did not have the advantage of knowing these few decades of opening theory that have developed, aided by modern engines, since his retirement. These concrete lines, variations and details are the difference, not some vague claims of "inventing a new style of chess" or "rule independence", which are total nonsense.
I'd second that this answer needs a book (I'd read it). Any new edition though is bound to summarize all of the beforeanswers, which I'll call "the classics", and incorporate what AlphaZero "invented" as a new style of chess, which may very well go along the lines laid out above.
These last five years however have shown that the matches between it and e.g. Magnus Carlsen have spawned new schools of thinking. Carlsen employed "new" moves into his own playing since. Any answer to the questions very much ought to include these recent developments.
Since I lack serious playing skills myself (and just made this account to contribute) here is what I'd expect to be spelled out in it:
YiFan's answer is excellent, but I think one piece of knowledge that emerged with the advent of computers can be formulated concisely: we are now much more confident that chess is a draw.
Before, at top-level chess White often played the "most critical" line in the most topical opening, Black responded in kind, and they would debate whether white gets a winning advantage or not. Kasparov won plenty of games out of the openings, and one may say he was arguing, by his play, that chess is a win: "I have an advantage in the most principal line, and I also have an advantage in sidelines, because they are inferior."
Nowadays, while some lines were "refuted", the overall trend is that black can make a draw in many, many ways - most sound opening systems have a path to equality. Not only these old main lines are a draw, but also many of the "inferior" sidelines are drawn, in more than one way. So, nobody is trying to prove that they can get opening advantage against the best play, like Kasparov did in his time. It has shifted to surprising the opponent.
