Black’s king is lined up with his bishop on the f-file and also with his queen at d8. A skewer against the f8 bishop is possible with Qf4, but it doesn’t work because the bishop has plenty of protection. How about a skewer against Black’s queen? Notice that White has two ways to do it: Bg5+ and Qh4+. Bg5+ just loses the bishop to KxB, but Qh4+ is safe. We aren’t quite finished, though, because Black has a defense to consider: he can interpose a pawn on g5. This presents no real problem, however, as White merely plays Qxg5+ and restores the skewer. (By the way, if Black's queen had protection you wouldn't want to use this particular sequence; instead you would reply to g7-g5 with Bxg5+, letting the bishop take over the skewer with protection from the queen now behind it on h4. This way the capture of Black's queen would then be made with the bishop, and the sequence would be a winner even if the bishop were recaptured at the end.)