A bag filled with sand. No, really. There are three basic types/uses. 1) Used in a "hemp house" as counterweight for a flying system. These should be purchased from a reputable manufacturer. Link. 2) Used to hold anything down. Often on the bottom of a lighting boom base or scenery jack. Often made in saddle bag style and sometimes called "shot bags," as they are filled with lead shot rather than sand. Dead Baby Seals: Easy way to make a sandbag. Take a tire inner tube, Zip-tie one end several times, fill with sand, zip-tie other end. Result is a 10-25 lb. sandbag that looks like a dead baby seal. Not for overhead or Fly System use! 3) Used to keep flood waters from invading. These are made of a synthetic burlap material and leak sand, and have no place in the theatre.