Bishop and rook pawn on the seventh hold against a rook when the attacking king is stuck in front of the pawn: the bishop protects the pawn and the king protects the bishop, and the rook cannot create a threat.
A classic fortress for the side with the minor piece. Black's pawn on a2 is protected by the bishop on b1, the bishop is protected by the pawn and the king on b3, and White's king on c1 is the only thing preventing a1=Q. White's rook can check from the a-file or the b-file, but Black's king stays on a3 and b3 and the structure never changes.