Once the pawn stands on the fifth rank, cutting the defending king off by a single file is enough to win. The attacking king walks forward and uses the squares beside the pawn to escape the frontal checks.
The pawn is on d5 and the black king is cut off on the f-file by the rook on e1. Black has the frontal defence set up with the rook on d8, but with the pawn on the fifth rank the defence no longer holds.