In addition to Qi, acupuncture and Traditional Chinese Medicine recognize a subtle energy system by which Qi is circulated through the body. This transportation system is referred to as the channels or meridians.
To better visualize the concept of Qi, and the meridians, think of the meridians as a river-bed, over which water flows and irrigates the land; feeding, nourishing and sustaining the substance through which it flows. (In Western medicine, the concept would be likened to the blood flowing through the circulatory system.) If a dam were placed at any point along the river, the nourishing effect that the water had on the whole river would stop at the point the dam was placed.
The same is true in relation to Qi and the meridians. When the qi becomes blocked, the rest of the body that was being nourished by the continuous flow, now suffers. Illness and disease can result if the flow is not restored.
Acupuncture is one tool used to restore the flow of Qi, by inserting needles into the acupuncture points (located on the meridians). These insertions are said to clear any residing blockages, or dams, thus freeing the river to better feed the body in its entirety.