A friend told me what is clasp（pearl clasp in detail)

A clasp originated as a finding for strands of pearls, although now they may connect a single strand or multiple strands of pearls, beads, or jewelry chain.
A pearl clasp has at least two parts - a body or "box", plus an insert that locks into the body, sometimes called a "bracelet tongue".  The body may be simple - a polished rectangle with at least one jump ring on the end opposite the bracelet tongue - or it may be very fancy with filigree patterns or corrugation.  The clasp's insert may be a hook that needs to fit around a pin before locking into the body or it may be a folded insert pushed directly into the clasp.  The number of jump rings on the bracelet tongue should match the number of jump rings on the body.
Some pearl clasps also have additional safety provided by a figure eight and a pin.  The figure eight, which looks more like a pop bottle, attached to one side of the clasp will click over a pin fastened to the second side of the clasp.