Base layers are the base of any layering system. They are designed to sit next to the skin and wick moisture away from your body. They do this by absorbing moisture (sweat) from the surface of your skin then allowing it to evaporate away.
By wicking sweat away from the body, a base layer keeps you dry, comfortable and chill free. Thermal base layers can also help to keep you warm by trapping an insulating layer of warm air around the skin.