Used to shoot arrows of love
A Cupid Bow is a magical bow used to shoot arrows of love, which the bow comes equipped with. One arrow can spread love and if an arrow hits a demon, the love it emits can weaken him. Presumably all Cupids are given a bow, however, Coop is the only Cupid shown using one.
When Piper, Phoebe and Coop were preparing for the battle against Neena's demonic army, Coop got his old Cupid bow. He told Phoebe that he would use it to weaken demons with arrows of love, so she could finish them off. He mentioned that he hadn't used it in a while, but still remembered how to use it. During the battle, he used the bow to fight off demons, as Sam and Henry carried Paige's body to her sisters so that they could make her whole again.