its a anchors. its like a drywall anchors that you use on drywall.but the anchors in that pic is for wood. you anchors that in the wood and drive a screw in it.
Wood Anchor Inserts, you can get them from any major Hardware store like lowes, HD, Ace, ect. I use those anytime I do plexi on an enclosure, they work awsome and done have a large head that you can see like T-nuts. Although we did use some Titanium T-nuts, with titanium allen head screws, way back in the day to secure our DD's in the wall, they were quite impressive!