This is a black plastic HFV Audio 8 inch subwoofer box adapter ring spacer. This adapter is for when your box cutout is to big for your sub. This spacer works on box cutouts up to 8" and the ring reduces the hole size down to 7.125".
 
We can also custom make any type of adaper rings up to 12" speaker size just let us know if there is somethinig specific you need.