White’s rook on f1 is blocked by his pawn on f4—a common sort of occurrence. What makes it interesting is that the pawn has a way to clear the file and threaten Black’s queen: f4xg5. To make this an effective double attack, of course, White needs a target for his rook once the pawn moves off the file. We look for ways to involve the enemy king, especially when it is so close to the relevant line. Can it be moved from e8 to f8 or f7? The obvious way to move a king is with a check, but here White has none. Another way is to plant a piece on the square where you want the king to move—making either a threat or a capture of your own in the process—and invite it to capture. The one way White can do this is with Nf7. This forks Black’s queen, bishop, and rook (the latter piece is loose), and so requires Black’s immediate attention. His only way to capture the knight is KxN, after which White has the discovered check f4xg5, winning the queen with the pawn whether it stays where it is or moves to f6 to block the check.
Now suppose Black grasps this and so tries to avoid KxN; does he have anything else in reply to the fork White launches with Nf7? Black would like to move the queen out of danger and protect the two loose pieces threatened by the knight. This can be done with Qf6. But then White still wins the exchange with NxR.