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, from a small 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 edition. London. Oxford University Press, 2005, pages 405-410.