Usually a metal circular screen placed in directly front of a Movie Light to “soften” the bright light source.  Scrims usually slip into clips built onto the front edge of the lights, or they are held in place by a C-Stand.  As there is such a wide variety of scrims, the ones that are being frequently used are kept in a canvas “Scrim Bag” and hang from the appropriate Lamp on the set.  The others are kept nearby on the Electric Department’s Taco Cart.