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 is dreaming of playing Rh5-Rh1 checkmate, but White's knight is blocking the path. Can you find a sacrifice to clear the way for Black's rook?
Black captures the knight and White makes the mistake of accepting the sacrifice. Even if White realizes now of the mistake it is too late for the King, there's no escape.
Now White tries to distract us by threatening checkmate. Let's end the game immediately and deliver checkmate first!
Checkmate.