Black’s knight is on a dark square. So are several of White’s pieces, most usefully his king and e1 rook, which can be forked from f3. But notice as well that f3 appears to be protected by the rook on e3. So examine the rook and its freedom of movement, playing through its move and what would then be possible in your mind’s eye. If 1…Nxf3+; 2. RxN—and then Black can play RxRe1. White’s queen wouldn't then be able to recapture at e1 because Black would have a second rook still trained on the square. The point: White's rook on e3 is pinned—not to its king, but to the other rook at e1. One way or another Black gains a pawn and the exchange. (Capturing a rook in return for a bishop or knight is known generally as “winning the exchange.”)