A wig-wag is the red light (also known as a "red-eye") near each door of a motion picture sound stage

Sound stage

In common usage, a sound stage is a soundproof, hangar-like structure, building, or room, used for the production of theatrical filmmaking and television production, usually located on a secure movie studio property.-Overview:...

. It flashes to indicate that cameras are rolling inside the stage and all people and vehicles outside should remain quiet. A flashing wig-wag is accompanied by one long buzzer when the cameras start rolling, and two shorter buzzers when cameras stop rolling.