A bishop whose own pawns stand on its colour is a bad bishop: it is tied to defending them and has few targets. The side with the good bishop fixes those pawns, attacks them, and uses the king to invade.
Black's pawn on d5 sits on a light square, exactly the colour of Black's bishop. That makes the bishop a nurse rather than a fighter: it must guard d5 forever while contributing nothing elsewhere. White's d4 pawn is on a dark square and can never be attacked by the black bishop, and White's f5 pawn is mobile.