Did you notice that both the whitequeen and the white bishop arepointing toward the pawn at g7?Black has nothing protecting thatg7 square except the king and it isoutnumbered by those two whitepieces. After the queen capturesthe pawn at g7, it is checkmate,for the bishop protects that queen,so the king cannot capture it.
Did you consider moving the white queen to b8? It wouldnot be safe there, for the black knight would capture it.