Hex nuts are six sided making them easy to tighten with a wrench or socket. They are used in conjunction with a bolt, threaded rod or other threaded fastener. The nylon insert compresses around the threads of the bolt/screw greatly increasing friction causing the nut to lock around the screw. This causes the nut to not come loose but still able to be undone with a wrench. Excellent for making sure the nut doesn't come off!