The presence of Black’s king and queen on the same file cries out “rook pin.” You have a rook you can use to do it: the one on e3, which can be transferred to g3. The remaining question is whether the pinning square is safe—not only from Black’s queen, but from the pawn on f4. On inspection, it is: the rook has protection against the queen from the knight on h1, and once the rook moves to g3—which must be carefully imagined—the pawn on f4 will not be able to capture it, because the pawn will become pinned to its queen by White’s bishop on d2.

That is as much as this example is meant to show, but the probable result after 1. Re3-g3 is 1. ...QxR; 2. NxQ, f4xN; 3. Rf1xRf8+, BxR; 4. Qe5—forking the Black pawns on c7 and g3, and making the capture on g3 a move later. White has won a queen and a pawn in return for a bishop and rook. (Another possibility is that Black replies to 1. Rg3 by interposing his d7 bishop on g4 to disrupt the pin. Now there's a big round of liquidations in which White ends up ahead by a piece.)