Black’s queen is loose and is about to get taken by White’s queen. He can retreat the queen or play QxQ; and QxQ would be a coup, rather than a wash played for defensive reasons, if he could first get rid of White’s rook on a1. He has a way to do this: he can drop a rook to the back rank with Rb1+. This gains him time; White has to save his king rather than play QxQ. But White isn’t required to play RxR and then lose his queen. Instead he can move his king to f2. But notice that now White's rook on a1 is attacked twice and defended just once; so Black can take it with QxR (if White replies QxQ, Black then has RxQ). It's another example of broad-mindedness on the back rank: if the enemy king moves, maybe there is other damage to be wrought there.

Ah, but not so fast. Taking White's rook actually is a mistake; Black has better. When the enemy king moves you religiously want to examine any checks you can give against it in its new position, even if that means interrupting a sequence you planned to play elsewhere. Here 1 ...Kf2 for White gives Black a new option against the king: 2. Qe1#.