A locket is a small decorative item that is typically worn around the neck on a chain or ribbon. Lockets are often made of metal such as gold or silver, and they typically have a small compartment that can be opened to hold a photograph or other small item. “Add locket” could refer to the act of attaching a locket to a necklace or chain, or it could refer to the process of inserting a photograph or other item into the locket’s compartment.
