Black’s queen is loose and under attack by White. He could move his queen or protect it easily enough, but instead he observes that White’s queen is guarded only by its king. If Black can give a check that forces White to put some distance between his king and queen, Black will be the one to play QxQ. Thus Black plays Rg1+, requiring White to play KxR—a decoy of the king onto a square from which it can't reach the queen. Black takes the queen next move.