There are several tactical clues on the board for White: the battery of rooks on the e-file; the kernel of a discovery created by the queen on c4 and the knight in front of it on d5; and the knight fork White can execute with Nc7. The key is to see how these ideas relate to one another. Imagine the forking move Nc7 and you notice that it also unleashes the discovered threat Qxf7#. Black can avoid this by playing QxQ, and that would quash White’s idea if all he then had was NxR, carrying out the fork. But still linking the various patterns on the board together, notice that Nc7 also forks the e8 square against which White exerts pressure with his rooks. Thus if Black replies to 1. Nc7 with QxQ, White then has 2. Re8+, RxR; 3. RxR# as the knight guards the rook.
This position combines three themes: the back rank mate, the knight fork, and the discovered attack. The crucial point to see is that White has two rooks lined up against a mating square, e8, and that Black guards the square twice. By adding another attacker against the square (his knight), White creates a mating threat on e8—while unmasking still another mate threat in Qxf7#. The latter possibility is what prevents Black from saving the day with NxN.
A theme to take away from these positions: a weak back rank can make a fine target for either of the pieces in a discovered attack. The attackers can land on the back rank directly; they also can create useful, distracting threats by adding pressure against a contested square on the back rank, thus threatening mate.