Nadi literally means river, channel, er vessel. It refers to the classification of the constitutional types in terms of vata, pitta, and kapha -- the three doshas dealt with in Ayurveda. A balanced functioning of the three doshas constitute the healthy and harmonious function of any system under consideration, be it on the level of psychology, physiology, sociology or ecology. The cosmos in which we live, provides a stable basis for the harmonious functioning of all its sub-systems, in the sense that the three doshas are equally distributed over the 27 nakshatras.
They repeat themselves in the following sequence, in which the numbers refer to the respective stars.
Chandra, Lagna and Surya are the three main indicators for the personality and physiology of a person. The respective stars in which they are posited at birth convey to the native their respective doshas.
It makes an interesting and rewarding study to see how these doshas express themselves in a person's actual constitution. As in all considerations regarding nakshatras, the nakshatra of the moon (janma nakshatra) is generally found to be the most important. Matching them has always been one of the most important considerations in determining marriage compatibility. Ideally marriage partners belong to different nadis. Especially the combination of two pitta stars is not recommended. Two vatas or two kaphas are passable, if other factors grant relief. For details see Kalaprakashika and Muhurtha.