"Nyloc Nut, also referred to as a nylon insert lock nut, polymer insert lock nut, or elastic stop nut. This elastic insert resists turning.
The plastic insert is placed at the end of the nut and its inner diameter (ID) is slightly smaller than the major diameter of the screw.
The insert deforms elastically over the threads of the screw, but threads are not cut into the nylon. The nylon insert locks the nut by applying a compressive force against the screw.
The nylon material located at the top of the nut prevent loosening from vibration or back off."