The salient visual point is that Black’s king, bishop, and rook all are lined up on the same diagonal. There is work to do before this can be turned into an effective fork, but the idea that will motivate your inquiry is clear. Bxc5 attacks the Black rook and aims the bishop at the Black king. The Black bishop on e7 is the only obstacle to a winning fork: it guards the forking square and blocks the line to the king. As usual, the remedy is an exchange of the bishop for a better target. 1. RxB, KxR; 2. Bxc5+ forks king and rook and again wins a piece and a pawn.