This is an assembly made (usually) of 36" of 1/8" wire rope and a rated locking clip, used to keep a lighting instrument from falling to the ground should there be a failure in the pipe clamp used to attach the instrument a pipe. It is wrapped around the yoke of the instrument (or affixed to an integral attachment point) and the mounting position (pipe/truss, etc.), allowing slack so the electrician can focus the instrument. They are a MUST HAVE for all lighting instruments.
Photo from https://www.fehr.com/lighting-safety-cables .
Photo from https://www.fehr.com/lighting-safety-cables .