Sometimes you can't capture a piece to remove a defender, but you can distract that piece. Deflection is the tactic of removing a defender by making it move to another square. Often this involves a sacrifice that will be worth it for checkmate or greater material gains.
In this position both players have a rook staring at each other but the only defender of the white rook is the king, black needs to find a way to remove that defender.
Black sacrifices a bishop to deflect the king from defending the rook, and will win the rook.
