Decoy is a tactic where you force a piece to a square that it doesn't want to be on. This can be used to remove a defender, lure a piece into a fork or even lure a king into checkmate.
Not all decoy tactics result in checkmate, sometimes material can be won.
White's bishop threatens the Queen by moving to b5, but allows white to fork the King and Queen in the next move.
Knight c7 check, forces the king to move and allows white to capture the Queen..