This stands for transcutaneous electrical nerve stimulation. Some hospitals have TENS machines. If not, you can hire your own machine.
TENS has not been shown to be effective during the active phase of labour, when contractions get longer, stronger and more frequent. It's probably most effective during the early stages, when you may have lower back pain.
TENS may also be useful while you're at home in the early stages of labour or if you plan to give birth at home. If you're interested in TENS, learn how to use it in the later months of your pregnancy. Ask your midwife to show you how it works.