Nitric oxide is a gas that is naturally produced in the body. We only discovered this gas was produced by the body in the early 1980′s but it is now regularly taken as supplements by bodybuilders.
The gas helps the blood to flow around the body and increases circulation. When it is taken as a supplement it allows the blood to flow faster around the body and gives the muscles more energy. So they will recover faster after working out, allowing the bodybuilder to train them again resulting in larger muscles faster than without taking the supplement.
Be sure to do your research if you are thinking about taking a nitric oxide supplement as they are not all the same. Some are more potent than others and contain more amino acids. Click Here to find Nitric Oxide Supplements that meet your needs.
This video explains exactly what nitric oxide is: