This move effectively pins the black bishop on h7. If the bishop moves, then Rh8# is checkmate.
The point of this move, Rh4, is that white will next play Bd3! followed by Rxh7 & Rh8#.
Black is helpless against the threat. Here is a few variations that prove the point:
- 1.Rh4 Be4+ 2. Kg3 Bh7 3. Bd3 Re4 4. Bxe4 Re8 5. Rxh7 Rxe4 6. Rh8#
- 1.Rh4 b5 2. Bd3 Re4 3. Bxe4 Bxe4+ 4. Kxe4 Re8+ 5. Kf3 bxc4 6. Rh8#
Black could try to give back some material with 1… Re4, but this leads to the variation:
1… Re4 2. Rxe4 Bxe4+ 3. Kxe4 b5 4. d6! (and now black can’t stop the advance of the pawn since white’s bishop controls the d8-square) b4 5. d7 Rb8 6. d8=Q+