A decoy occurs when a piece is decoyed onto a fatal square.
Let's look at the following example:
White plays 1. Bxf7+ decoying the Black King and after 1...Kxf7 2.Qxd8 wins the Black Queen!
Decoys are often used with other tactics such as knight forks as in the following example:
Black wins by playing 1...Rh1+ (decoying the King) 2.Kxh1 Nxg3+ (Forking King and Queen) 3.Kg2 Nxh5
Decoys are also often seen with checkmate in mind as in the example below. Can you see how White achieves checkmate using a decoy?
White decoys the King with 1.Qd8+ and then after 1...Kxd8 there follows 2.Bg5 Ke8 3.Rd8# (or 2...Kc7 3.Bd8#)