A pin is an attack on a piece that is unable to move away. Should it move to safety, a piece behind it will be taken.
Sometimes it is just not smart to move because you would lose the strong piece behind it. When a piece is pinned to your king, it simply cannot move. Doing so you would put your own king into check, which is against the rules. Pins that are targeting the king are absolute pins.
Pins always involve three pieces
1. the attacker
2. the attacked piece in the front
3. the piece behind it that will be at risk when the pinned piece moves
What do you have to do?
Pin one of your opponent’s pieces and win material.