Black's knight is on a dark square. White’s king and bishop also are on dark squares, creating a potential fork at f3; but f3 is protected by the queen at d1. Can the queen be captured? No. Is the queen protecting any other piece that can be attacked? Yes, the bishop at d4—and the bishop already is one of the pieces in the fork. So if 1…QxB, 2. QxQ, you not only have a clean fork at f3; you have a better one: Nf3+ wins the queen.