A sacrifice in chess is when you momentarily give up a piece in exchange of a future outcome, probably taking another piece, gaining a better position, or start cooking a checkmate. Sometimes you know what you are getting, sometimes you don't.
Don't be afraid to give up your pieces if it ultimately leads to winning the game. Pay particular attention to checks, captures and threats that sacrifice something for checkmate, future material gains or a strong attack.
Some times the sacrifices don't deliver a gain immediately but you there will be. You can't see the final outcome but you can sense it. Black is attacking full force due to the position of the pieces and attacking directly the King side.
Maybe there's a way to open the king's pawn line but white can't really see it, just has a general idea that breaking those lines can get the heat off, a sacrifice must be done.
By moving the bishop to h7 we force the king to take the sacrifice and get out in the open or to move to h8, where it will be later on under fire and would have lost that bishop, so what would you do?
Whatever the King does at this moment is not relevant, the checkmate is almost done because we have the Queen on the same file as the King right after his move.
Mate secured.