For a pawn on the fourth rank or further back, the key squares are the three squares two ranks ahead of the pawn. For example, a white pawn on e4 has key squares d6, e6, and f6. If the white king can reach any of these squares, the pawn will promote by force.
For pawns on the fifth or sixth rank, the key squares shift and the rules become more nuanced. A pawn on the sixth rank has key squares on the seventh and eighth ranks. Rook pawns (a- and h-pawns) are special cases where the key squares are more limited due to the edge of the board.
Knowing the key squares allows you to make quick assessments in pawn endgames without calculating long sequences of moves. If you can determine that your king can reach a key square, you know the pawn will promote. If the opponent's king controls all the key squares, you know the pawn is stopped. This knowledge transforms complex-looking endgames into clear evaluations.