With the pawn on the seventh, the attacking king in front of it and the defending king cut off by one file, White wins by building a bridge: the rook goes to the fourth rank and later shields the king from checks.
The single most important winning position in rook endings. White's king stands in front of its own pawn on d8, the pawn is on d7, and Black's king is cut off on the f-file. Black's rook on a2 is waiting to check the white king as soon as it steps out.