In the above position the Key Squares are d5-e5-f5. Think of key squares as the 3 squares, 2 rows directly in front of the pawn. This changes if its a rook pawn, or the pawn is on the 6th (3rd) rank.
A key square (also known as a critical square) is a square such that if a player's king can occupy it, he can force some gain such as the promotion of a pawn or the capture of an opponent's pawn. Key squares are useful mostly in endgames involving only kings and pawns. In the king and pawn versus king endgame, the key squares depend on the position of the pawn and are easy to determine. Some more complex positions have easily determined key squares while other positions have harder-to-determine key squares.
What the heck is a key square? Obviously I'm new to this but it shouldn't be that difficult to figure out.