An anal hook is a a metal hook with a loop on one end and sometimes a ball on the other end. The loop is on the longer end of the hook and the shorter end is inserted into the anus. An anal hook is usually about one inch thick and six inches long.
The purpose of the anal hook is to hold a submissive partner in place during a BDSM scene. While the short end of the hook is inserted into the anus, a rope is threaded through the loop on the long end. That rope can then be attached to an overhead fixture which would hold the wearing in position with their rear in the air or it can be used to bind their wrists.