TENS is a way of controlling pain through the gate theory where, it is believed, selective stimulation of certain nerve fibers blocks signals carrying pain impulses to the brain.
Electrodes emanating from a hand-sized battery-powered device are strategically placed, and the patient instructed in its use, usually by a physiotherapist or physician.

Bercovitch M, Waller A. Oxford Textbook of Palliative Medicine, 3rd ed. London. Oxford University Press, 2005. pp. 405-410.