By now this one should be easy. Begin with the usual reconnaissance of enemy pieces, looking for any that are unguarded or underdefended; here it turns up the Black knight on e3. White can’t give check and attack the knight at the same time, so he looks for the makings of a mate threat: another piece that attacks a square next to the king. Once more his bishop answers the purpose; it attacks g7. So he looks for ways his queen might attack g7—the mating square—and the loose Black knight. He finds Qd4, which wins the piece.
As these first examples all show, a bishop aimed at squares next to the enemy king can be a valuable resource, as it creates a fertile climate for double attacks with the queen. The mating threats it supports may be simple to defuse (here the simple pawn move f6 puts out the fire for Black), but of course the purpose of the exercise never was to deliver mate. It was to take loose enemy material.