You see the pin: Black’s rook is stuck on c2; White threatens to take it next move with his bishop. Black’s only hope is a move that both gives check (and thus requires White to do something other than play BxR) and disrupts the pin. Black has nothing he can use to attack the pinning piece or force the king to move, but another way to dissolve a pin is to block it. The idea also comes into view clearly enough if you are insistent about inspecting every check you have to offer—not just with each of your pieces, but with your pawns. Here Black has this one: f7-f5. It blocks the diagonal on which the pin is imposed, and after White defends with Rxf5 the pin on Black’s rook is gone.

Incidentally, what is Black’s next move? Again the answer is easy if you have a look at every check you would be able to give, for here there is only one to worry about: now that the rook legally can move, Black can play Rg2—mate.