Acupuncture is an alternative medicine methodology originating in ancient China that treats patients by manipulating thin, solid needles that have been inserted into acupuncture points in the skin. According to Traditional Chinese medicine, stimulating these points can correct imbalances in the flow of qi through channels known as meridians.
The best way to describe meridians is to picture rivers of energy flowing throughout the body irrigating and nourishing the tissue & organs of the body. When an obstruction occurs (think of a dam in a river) then the flow of energy is slowed down or even blocked thus causing a shortage to the target tissue and a back upflow occurs to others.
Meridians are influenced by needling of the acupuncture points. The acupuncture needles unblock the obstruction and thus re-establish the energy flow.