A hanger bolt is a furniture part. A special tool is used to drive them in. They are machine threaded on one end to take a nut and have a lag (wood) thread on the other. This enables a threaded stud to stick out of the wood and for additional items (often wood as well) to be fastened in place. (If you don't wish to use the tool, you can always thread a nut on and use a standard socket. They come steel (plain and zinc), stainless, and brass. Metric sizes are made to order and require minimum quantities. They come both plain center (there is an unthreaded part in the middle) and fully threaded (the threads meet in the middle as in the photograph).