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.
Black's king looks safe in the corner, but can you find a powerful sacrifice that forces the king out of its protection?
White forces the Black king on the same diagonal as the White bishop, now we can advance the pawn and check the King at the same time.
Black blocks with the pawn because moving to the king to h8 would allow fxg7 checkmate. Can you find a forced two-move checkmate?
Now it doesn't matter that black has a Queen and a rook because checkmate is imminent.