Black has just used his knight to capture your bishop on g3. This was a mistake. By leaving his knight on g3 Black provides you with a capture you can use to move your own pawn off the h-file and leave your rook with a clear path to h7. (Be alert to moves that allow pawns to capture and thus open the lines on which they used to be stuck.) The important thing is to resist immediately recapturing and to consider instead whether you can make something larger out of this opportunity. You see that Black’s f-pawn has advanced, so thoughts of Greco’s mate come to mind; if you move your knight, your bishop will bear down on g8. But pick the knight's move carefully, for it is not merely an obstruction. The h-file needs to be opened on Black’s end, and your rook on h1 can’t do that by itself. It needs another piece to help, and the other piece is the knight. So the first move is 1. Ne7++, a double check that requires Black to move his king to h8. Now you can pry the Black pawn from h7 with the familiar 2. Ng6+. Since Black’s king has no flight squares he is forced to play h7xN—and then White discovers Greco’s mate by at last playing h2xN.