A rook in front of its own a7-pawn is a poor piece: it can never leave a8. The defender draws by keeping the rook behind the pawn on the a-file and the king on g7 or h7.
White's rook is on a8 in front of the a7-pawn and Black's rook on a1 is behind it. This is one of the most common structures in rook endings, and it is a draw as long as Black's king stays on g7 or h7.