TENS (Transcutaneous Electrical Nerve Stimulation) unit is a small device that uses low-voltage electrical currents delivered through electrodes placed on the skin to provide pain relief by blocking pain signals to the brain or stimulating the body's release of natural painkillers (endorphins). It works by sending gentle impulses along nerves, essentially "scrambling" or overwhelming the pain signals with sensory input, offering relief for conditions like chronic back pain, arthritis, and nerve pain.