What Black pieces are loose? All of them. White has no way to check the Black king and attack any of those pieces at the same time, so he looks for a mating threat; he asks what other pieces he has attacking squares near the king. This time his bishop isn't helpful, but his knight is: it attacks g7. If White’s queen were to land on that square, it would be mate. So White looks to move his queen to a square that would allow it to attack g7 and also attack one of the loose Black pieces. This leads to Qg5, which wins the rook after Black makes a move to protect against Qxg7#.