Nadis are flows of energy in the body which in modern day times have been translated as nerves. Nadis are subtle channels through which the pranic forces flow.
Ancient texts say there are 72,000 nadis, ten of these are major sensory nadis with the three most significant being: Ida, Pingala and Sushumna. The most important of these being Sushumna.
Further Reading (Theory of Chakras):