Figure[White to move]

The key to this position is that White has a check he can give with his queen by moving it to e5—and from there the queen also is lined up against a pair of Black pieces on the fifth rank. Obviously Qe5+ isn't yet feasible because Black’s rook protects the square, but the potential for a fork should attract your attention and cause you to focus on getting rid of the rook. There are standard tools available for the purpose. A first recourse is to capture the nettlesome piece, but White can't; a second is to take something the rook protects, but this, too, is impossible. There remains a third option, however: threaten the rook with something less valuable than itself so that it has to be moved rather than just protected. A pawn is best for such purposes, so White plays the simple g3-g4. If the rook moves to safety, White has the queen fork Qe5+, winning a piece.