Black’s situation looks rather desperate: his queen is about to be taken, and if it moves White is posed to mate with Rxg7 and a couple of simple follow-up moves. Black has no checks to give, but it always is worth looking at any mating threats that might be available. You should be especially conscious here of the weakness of White’s back rank: his king is trapped in the usual way and has no defenders at all. Black thus wonders whether he can get one of his two heavy pieces there or at least threaten to do so in a way that will win something else. For now the implication is that the d4 pawn is pinned. The other point to notice is the relative pin that Black imposes on White’s rook: if the rook moves, White loses his queen. The way to exploit these ideas is by putting a piece en prise to the pinned rook and also to the d4 pawn—and in a position to mate on the back rank. All this Black does with Rc5, threatening Rc1#. Think about White’s reply options:
(a) He can’t interpose anything, so focus on captures or ways he might make his king safer.
(b) He can't capture the rook with d4xR; this permits Qd1#; since as we know the d-pawn also is pinned to the back rank.
(c) He can capture the rook with RxR. This forfeits the queen to QxQ, but still is his best bet.
(d) White’s last option in reply to Rc5 is to move his g-pawn to g3 to give his king a flight square. This loses a whole rook, as Black then plays RxR; if White replies QxR, Black has QxQ and his queen is the only piece left on the board (setting aside the kings, of course).