TENS stands for Transcutaneous Electrical Nerve Stimulation which is predominately used for nerve related pain conditions (acute and chronic conditions). TENS machines work by sending stimulating pulses across the surface of the skin and along the nerve strands.
The stimulating pulses help prevent pain signals from reaching the brain. Tens devices also help stimulate your body to produce higher levels of its own natural painkillers called endorphins.
For more information read how a TENS unit works.