(As I understand it) according to Traditional Chinese Medicine, energy (chi) flows through the body and we have to maintain this flow of energy. If there is stagnation of this chi, then illness occurs, which is why we will go to an acupuncturist. You can encourage the flow of chi or get stagnating chi to move again by performing exercises such as chi kung and tai chi but you can also stimulate chi to flow by sticking needles in specific points. These points lie on meridians through which chi flows.
Whether you believe this or think it's bunkum is another matter.