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 bishop attacks a pawn on a4, but is dangerously placed. Can you find a sacrifice on the next move that will gain material for White?
White sacrifices the Rook for the Bishop but forces the Rook into a fork position with the King in the same diagonal that white's bishop can access to.
Black's king and rook are forked, so White will regain the sacrificed material and be up a bishop.