Black’s rook masks his bishop on the long h1-a8 diagonal. The question is whether the king can be drawn into the line of fire. The usual way to so move a king by checking it with another piece; thus here Black can play Qg2+, requiring KxQ. Since this involved sacrificing a queen, you seek mate as a payoff; Black wants to move his rook where it can add to the pressure on White’s king (now on g2) that will be created by the unmasked bishop on b7. The rook can go two places to do this, both involving a capture: Rxf2 and Rxg3. The move to f2 is unproductive since White just replies KxR. Rxg3, however, is checkmate: it is double check, so capturing the rook with a pawn does not help White; meanwhile the rook has protection against White’s king from the other Black bishop on d6, and it cuts off the only squares the king could use to escape the diagonal.