A knot of pieces like those surrounding White’s king can create intricate problems. But before getting buffaloed by the intricacies, try the standard method of considering checks and consequences. Here Black has two to ponder. The first is Bxb2+, which ends up trading the bishop for a rook after White responds RxB and Black replies a3xR+. Black's second idea is to start with a3xb2+. In reply to this White can’t move his king and can’t interpose anything, so he has to capture the pawn with his rook—which then is pinned by the bishop on f6. What do you do with a pinned piece? Attack it again. Here the only resource that remains for Black is his king, but it’s good enough. He plays Ka3, and has Bxb2 next move. This wins the rook; more to the point, it leads to mate. (White can take the c2 bishop with his f2 rook, but this just forestalls the inevitable.)

Of course you might as well have seen the idea here by noting the pin of the b2 pawn from the start. You upgrade it to a rook with a3xb2, then hit the rook with Ka3 and BxR.