It's the "load space divider" or "safety net". I got one of these way back - pretty handy when carrying bulky stuff in the back. It has 3 hooks that engage the tie-downs on the back of the rear sets just behind the headrests. Can be used with the seats foled down (net is full open) or with the seats up (net half open).

Will post a pic the next time I have it in the car.

- Rob