With bishops of opposite colours, a single extra pawn is a draw whenever the defending king can reach the square in front of the pawn or the defending bishop controls it from a safe diagonal.
Opposite-coloured bishops are the defender's best friend. Here Black's king sits on d5, directly in front of the pawn, and the white bishop can never attack a king on a dark square because it travels on light squares. The blockade is unbreakable.