I did some research (Google), but it still unclear to me on exactly what a pawn break is.
I was reading this question: The theory of pawn breaks
It is about the theory of pawn breaks, but before I could understand this, it would be helpful if I actually knew what a pawn break is.
If someone can explain it simply and with some diagram(s), that would be great.
A pawn break is a pawn move designed to free the player's position.
Generally speaking, a pawn move is only called a pawn break when the moving pawn is on a file adjacent to two enemy pawns facing each other, and the pawn moves forward to the same rank as the player's other pawn.
Since a picture is worth a thousand words...
[FEN "8/8/8/4p3/3pP3/3P4/2P2P2/8 w - - 0 1"]
White can play either 1. c3 or 1. f4 and these moves would be considered pawn breaks because they attack a black pawn that is blocking a white pawn (the d and e pawns respectively).
1. f3, however, is NOT a pawn break because it does not attack an enemy pawn. Similarly 1. c4 is not a pawn break, even though black could capture the pawn with 1... dxc3 e.p. and get to the same resulting position because again, it does not attack a black pawn.
...f6 in the French Tarrasch is a perfect example of a pawn break that doesn't sacrifice a pawn.
A pawn break is a "clash" of pawns that results in an exchange of pawns. For instance, if White has pawns on d4 and e4, and Black has pawns on d6 and e6, White's move to e5 (or d5) might cause a pawn break if Black elects to exchange. If Black plays d5 in response to e5 or 35 in response to d5, he avoids a pawn break.
For what it's worth, when I asked the question you mention, I had in mind a position like this (black to move):
[FEN "rnbq1rk1/pppp1ppp/4pn2/8/1bPP4/2NBP3/PP3PPP/R1BQK1NR b KQ - 0 1"]
[startflipped ""]
The question regarded whether black should push the c-pawn or the d-pawn, or push neither, or prepare something before pushing. Of course, the question did not regard this particular position, nor was it specific to the opening or to the midgame; but it regarded rather the principles that guide the black player to open files from such positions generally.
Update: In response to helpful comments: I do not know for sure how the term pawn break is conventionally defined. I should defer to @Andrew's and @TomAu's answers. Nevertheless, pawn break is the term I seem to have heard opponents use for the act of pushing a pawn with the intent to force a file open.
Thus, to answer the question directly, to the extent to which my terminology is correct, d7-d5 would constitute a pawn break in the position diagrammed, because the likely and intended effect of such a pawn move would be to open, or half-open, one or more files by trading off the pawns that occupy the files.
In his excellent book The Giants of Strategy Neil McDonald proposes a slightly different definition of pawn break. Reviewing games of Capablanca & Nimzowitsch he defines pawn break as "putting another pawn alongside your most advanced pawn" (p 111). An interesting idea, although narrower and somewhat different from other answers.
