Black has worries; he is up a piece (though White has an extra pawn), but his queen is pinned by White's rook and seems about to be lost—and his knight on f4 is hanging (in other words, it is under attack and has no protection). Look for counterplay before falling into a defensive mindset. Black’s knight and White’s king and queen are on dark squares, suggesting a possible fork at e2. Alas, e2 is protected by the knight at d4. You look for ways to take out the knight directly but find none. So now look more closely for anything else the knight protects that you might take. It guards the rook at c6, which you can capture with your queen—extinguishing the pin as well. So 1…QxR; 2. NxQ, Ne2+ and the fork is made. Black wins the exchange, having traded queens and swapped his knight for a rook.
Once you see the relationship between the rook on c6 and the potential fork far away on e2 (they are connected by the knight on d4, which guards against both of them and thus is overworked), you can choose between exploiting the situation at either end. In other words, you can (a) play the sequence just described, or (b) start with Ne2+, allowing White to play NxN, and then playing QxR since the White rook’s guard (the knight) has abandoned its defensive duties. Which sequence is better? Either way you end up ahead the exchange, but the first sequence also has the advantage of getting the queens off the board—which magnifies the significance of Black’s material edge.
That last idea is worth another minute of explanation if it's not already familiar. In simplest terms, the point is this: if you have one piece and your opponent has none, this lets you dominate the game in a way that an edge of five-to-four doesn't. So if you win a piece from your opponent and have five pieces against his four, your usual goal is to exchange away the rest of the pieces on the board, making captures when you can; finally you are left with the only attacker (at which point your opponent probably resigns, if not sooner). Naturally it follows that if you can start whittling down the number of pieces on the board while you're gaining your edge in the first place, you want to do so—as Black does here by starting with QxR. (Leaving White's queen on the board also has another disadvantage: it lets him go on to play Qg4+, forking Black's king and the b4 bishop. That pattern will become more familiar in the next section.)