A backward pawn is a pawn that can no longer be supported by a friendly pawn and stands on a half-open file, making it a structural weakness that is difficult to defend.
A pawn becomes backward when its neighboring pawns have advanced further, leaving it behind. It cannot safely push forward because the square in front of it is controlled by an enemy pawn, and no friendly pawn can come to its aid. The square in front of the backward pawn becomes an ideal target for enemy pieces, especially knights, which can settle there permanently. Note that this square is not necessarily a hole: a hole is specifically a square that can never again be defended by a pawn, a related but distinct concept.
A typical example: White has pawns on c4 and e4, but the pawn on d3 is backward. Its neighbors have advanced, d3 cannot push forward because d4 is controlled by an enemy pawn, and no white pawn can come to its defence. The d-file is half-open for Black, and enemy rooks and the queen will naturally occupy it to apply constant pressure on the backward pawn.
In practice, watch for backward pawn formations as early as the opening. If you have one, overprotect it with your pieces and look for the right moment to advance it and eliminate the weakness. If your opponent has one, place your rooks on the half-open file and station a piece — ideally a knight — on the square in front of that pawn.
