What do they mean by "key squares?"

Is it true that in some openings, there are some squares that are important for you, that are not so much important for your opponent? If so, where can you learn about this? And when a square is important, does that mean that you should be ON IT? Or attacking it.....in a sense. What's the deal with that? Thanks everyone. It really means a lot to me. I'm trying to get better in my chess. Bye :)

A key square is a square that is strategically (or even tactically) important.  I know you asked about the opening, but a simple way to illustrate what is meant by the term is an ending.  Imagine you are white and you have a pawn on e7.  The e8 square is now very important.  If you can guard that square successfully, you can now play e8=Q and likely win the game.  The e8 square is a key square in that situation.

In an opening, one might be looking to control (say) the d5 square if (for example) the opponent weakened it by pushing pawns past it, etc.  This square might be strategically important because (a couple of examples) it would be a great place for an advanced knight to be placed (difficult to attack/remove) or because controlling it causes the opponent to have a lot of trouble playing a freeing pawn advance.

An example of a "key square" is the square d5 in the Sicilian Opening.  But to understand why it is important you would have to understand the Sicilian Opening.

Key squares that you will understand now are the central squares (d4,d5,e4,e5).  They are important because they give your pieces access to all 4 corners of the board.

Thanks Yaroslavl. And pawpatrol, when your controlling that d5 square, the example you gave, when you say that in such a case, that, "it causes the opponent to have a lot of trouble playing a freeing pawn advance," are you talking about your opponent wanting to (in this case, let's say black) take his pawn down a square so it doesn't stay in one place? You know on the home square?.....?

There are others on this forum who can more effectively speak on this matter than myself, but I want to opine that chess is something of a zero-sum game, so logically if a square is key to your opponent, it is key to you as well.

An example which comes immediately to my mind is a game examined in GM Melikset Khachiyan's "Evaluation and Planning! - Part 1", Capablanca v. Alekhine. In that game, there were a pair of squares upon the respective fourth ranks of the a- and c-files for both players (e.g. white pawns at a3 and b4, black pawns at a6 and b5), and either could elect to control that sector of the board by posting a piece upon it. Much of Alekhine's middlegame plans revolved around taking control of these squares (primarily the c4-square) and bringing the battle to White's queenside, and Capablanca found himself overrun for not clamping down on these squares.

Also, in many lines of the Sicilian, the d5-square is important to both players primarily because of the following: achieving the d5-break as Black at least equalizes and often leads to activity in the center and queenside. Black looks to obtain this while White looks to deny this for Black. What's good for your opponent is bad for you.

EDIT: I see some others have already addressed the importance of the d5-square in the Sicilian. I apologize for reiterating.

I've found the game I mentioned above. You may find it useful to review it:

Key squares can change with every move - especially in the middle game. The best players find them almost every time........

Yes, that's what I mean by a pawn advance (could also be two square advance).  A freeing pawn move is one that helps your pieces (usually behind your pawns) become more active (more space to move around in, more open files and diagonals for them, etc.)

I am not sure I understand your question.  So, instead I will use a specific example.  After the moves 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6, the most important factor to note about the  position is that Black enjoys a 2vs.1 central pawn majority (White pawn at e4 and Black pawns at d6, e7).  Black has a material advantage in the center.  The extra pawn at e7 will eventually move to ...e6 giving Black an extra pawn to eventually make a ...d5 pawn break (this terminology you will understand later) which gains space for the Black pieces among other things.  Of course, the move ...e6 requires a tempo (time).  As you probably know (material, space, and time) are the only advantages/disadvantages in chess.  Further, every move in chess has advantages and disadvantages.