The defending rook sits on its second rank (here the seventh), keeping the attacking king off the sixth rank. The king stays on the back rank, and the rook checks whenever the white king steps forward.
The second-rank defence is the other main drawing method in rook-and-bishop versus rook. Black's rook on a7 controls the entire seventh rank, so White's king cannot reach the sixth rank without being checked; without the king on the sixth, there is no Philidor position and no mating net.