They are about 1/2 inch thick and turn into a 3/8 inch hole. For 10-24 or 1/4-20 machine screws. Commonly used to allow a machine screw to attach to wood so that it can be removed and replaced repeatedly without damaging the wood.

West sells them in stainless for about $10 each. I'd bet you could find them cheaper elsewhere.

http://www.westmarine.com/buy/fasco-fasteners--threaded-inserts-bulk-packs--P013_390_002_035