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.
In this position white is down a full rook but the game isn't over yet . Whites Queen prevents the black queen to move to the 7th rank.
So white lures the king to the g5 square by checking it, allowing white to checkmate in the next move.
