Black’s queen is pinned by White’s bishop on c4. Black can’t play QxB because the bishop has protection from the pawn behind it. This is a desperate situation, so think hard about what resources Black has available. First, his bishop on e4 pins the c2 pawn. This is important: it means the pawn, despite appearances, is defensively inert. If it weren’t there (in a sense it isn’t there), what would Black do? He would eliminate the bishop’s defender with check via Rxb3+. (Of course you might reach the idea as well by just examining every check you can give.) Since White would be in check he would have no time to play BxQ; and since the pin of his pawn would prevent him from playing c2xb3, he would have to try something else to defuse the threat to his king—e.g., BxR or Ka1. Either way White’s bishop is left unguarded, so now Black can play QxB after all. It's true that if White chooses BxR, Black loses the rook; but in addition to having saved his queen he ends up with a forced if longish mate. (His queen, bishop and pawns close in for the kill in six moves.)