Prevailing torque is the amount of torque needed to run a nut down a thread on nuts that are designed to resist self-loosening under vibratory forces. These types of nuts are known as locknuts or prevailing torque nuts, as opposed to free-spinning nuts.
Prevailing torque nuts fall into two categories: all metal nuts or nylon insert nuts. On most metal locknuts, the prevailing torque is created by distorting the top threads of the nut in some way. On locknuts with nylon inserts, the nylon adds extra resistance between the mating surfaces, which requires prevailing torque to be applied to overcome that resistance.