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.
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 :)