With the rook behind its own passed pawn and the enemy rook in front, the defending rook is paralysed. The attacking king walks across the board and wins if the defending king is too far away.
Tarrasch's rule in its purest form. White's rook on a1 stands behind the a-pawn; Black's rook on a8 stands in front of it. The black rook can never leave the a-file without losing the pawn race, and every time the pawn advances the rook has fewer squares. Meanwhile the white rook is completely free, but White does not even need it: the king does the work.