I just started playing chess one week ago. I've done a bit of reading but can't find anything that explains what is meant by openings with a lot of theory versus ones with a little or no theory. Can this be explained in a "chess for dummies " language?
In chess, "opening theory" or just "theory" means "established opening lines": usually lines that have been studied and judged to lead to more or less equal positions, and appear in books.
It's unfortunate terminology since it matches neither the day-to-day meaning of the word (something that's contrasted to practice) nor the meaning used in game theory. But we're stuck with it.
While "opening" sometimes refers to a specific line, it's better to think of it as a game tree, that is, a collection of lines with a common origin. Openings with "lots of theory" are those with large game trees. This is usually a combination of two factors: long lines (the tree is deep) and lots of variations (the tree is wide, with a lot of branches).
Openings that are popular among top players will be studied more so will tend to have longer book lines. Openings that lead to positions with a lot of good moves will have lots of variations, while openings with forced moves will naturally have fewer variations.
In short, openings with lots of theory are those with more pages in opening books.
For example, Mastering the Chess Openings devotes 75 pages to the Sicilian Opening, and only 10 to the Caro-Kann. So we could say that the Sicilian has more theory than the Caro-Kann. Of course, different books will give you different numbers, but you get the idea.
Openings with a lot of theory are those with extremely long book lines, especially if there are lots of viable moves at each point in the game. Here's an example of such a line. Ten moves into the game, White plays a subtle move order to provoke a non-obvious pawn move by Black, in order to get a slightly stronger initiative. If you didn't know anything about theory you would have little to no chance of spotting this subtlety, but if you do, then you can gain an advantage over those who don't.
The reverse is the case for openings with little theory. For obvious reasons people play moves such as 1.b3 a lot less than 1.e4 or 1.d4. As a result, 1.b3 is much less explored, and there is less theory.
"Opening theory" should be seen as the constant, somewhat-like-scientific search for a way to force an advantage for white in the opening. People interested in theory write opening books and opening articles that don't just repeat what was already published before, but that try to improve on the already known theory -- new tries for an advantage for white, new ways to equalize for black.
All those published works taken together (books, DVDs, articles, published analysis of games, et cetera) form "opening theory".
Some openings, like the Open Sicilian (1.e4 c5 2.Nf3 d6/Nc6/e6 3.d4 cxd4 4.Nxd4) have have lots of attention in the past, with piles and piles of books on it published. Many times it seemed white was better, always new ways for black to equalize were found. Whereas say the Ponziani (1.e4 e5 2.Nf3 Nc6 3.c3) has basically always been considered equal and therefore hasn't got much attention.
Practical players also use this theory to prepare these games. If the opening is very sharp, with many published lines leading to either checkmate or a draw by perpetual check depending on details you can't find with a few minutes worth of thinking, and your opponents may have memorized them -- then you better memorize them too, or find another opening. Chess players buy lots of opening books and try to memorize them.
But practical players also prepare and study openings not with the goal of getting an advantage, but with getting a playable game that they'll understand better. E.g. opening theory says that after 1.d4 d5 2.c4 e6 3.Nf3 Nf6 4.Nc3 e6, 5.cxd5 is bad -- black can play 5...exd5 and develop his pieces more easily than when the white is not on f3 yet, say with 3.Nc3 Nf6 4.cxd5. So you won't find much about that line in opening books, but strong players do play it! And they will win if they understand the resulting positions better than their opponent. It just isn't opening theory.
Naturally, the heft of opening theory must pay more attention to lines which generate imbalances. Quiet, balanced openings present less immediate concern (and fewer opportunities for opening theoreticians -- who depend upon asymmetric threats to narrow their path of study).
To illustrate with a simple example, consider the exchange variation of the Ruy Lopez (1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Bxc6 dxc6 etc). Why would white spend two of his first four moves to prematurely surrender the light-squared bishop for black's knight? The goal is to weaken black's pawn structure (if white manages to exchange all remaining officers, the pawn endgame is won).
It did not take long for opening theory to judge that black's bishop pair (and superiority on the light squares) provides more than enough resources to hold the balance (and the drawish nature of rook endings tends to mitigate the risk). That's why most white players do not capture on c6.
In fact, black players rarely play 3...a6 anymore, because the counter-attacking 3...Nf6 offers even more imbalances (read: opportunities to profit by opening study -- the ensuing "Berlin Wall Variation" has all but ended favoritism for the Ruy Lopez).
Today, computers can provide a instantaneous answer to most opening questions, and as a result, memorizing piles of computer output yields diminishing returns.
If you want to drive a Ferrari, be prepared to devote considerably more time/money keeping it up. Opening theory is just that -- a gathering of enthusiasts in a car garage, forever talking of ways to increase horsepower of their prized opening.