The Indian Yoga Nadis, like the Chinese Qi Meridians and the Thai Sib Sen, are channels or pathways through which vital life energy, called Prana in India, is distributed to the gross, subtle and causal body.
The Nadis (plural of Nadi) are an interconnected network of Prana energy channels (depending on the context, Nadis translates as canals, tubes, pipes, nerves, pulse, or blood vessels), which has special “distribution points” or “energy distribution centers” in the body, called the Chakras.
Although there are different interpretations, it’s generally assumed that the Nadis of the physical body (gross body) are channels that transport air, water, nutrients, blood and other bodily fluids, which could be translated as our blood circulatory system and our lymph system.
However, in the subtle body (pranic energy body) and the causal body (the undefined root or seed body of the both gross and subtle body), the Nadis are considered channels for cosmic, vital, seminal, mental, and intellectual energies, which is called Prana (life force).
Mind that there are other Indian classifications of “types of bodies” also, like in modern Kundalini Yoga, where it’s taught that there are ten spiritual bodies: the physical body, three mental bodies and six energy bodies, including an 11th embodiment which supposedly represents a pure state of non-dual consciousness.
In any case, the three main Nadis run from the base of the spine to the head, and are the Ida Nadi on the left side of the spine, the Sushumna Nadi in the center of the spine and the Pingala Nadi on the right side of the spinal column. These correspond more or less with the Sen Ida, Sen Sumana, and Sen Pingkala energy lines of the Traditional Thai Massage framework, which is heavily influenced by Ayurvedic and Indic healing concepts.
Although the Qi Meridians of Traditional Chinese Medicine (TCM) are based on the same concept of transporting and distributing Prana (called Qi or Chi in China), the trajectories Qi Meridians take are very different in comparison with the trajectories the Prana Nadis take in our bodies. Roughly said, the Prana Nadis are rather “function based,” while the TCM Meridians are rather “internal organs and Yin-Yang based.” Nevertheless, of course, either Indian or Chinese, finally it all comes down to optimal “functioning of body and mind.”
Depending on a variety of Indian scriptural sources, the number of Nadis (and sub-Nadis) is thought to be 101, 72000, or 350000, or sometimes even millions. Usually, 14 Nadis are considered specifically important, and among those, the three Nadis mentioned above are of most significance.
Also, it’s through the Sushumna Nadi the so-called Kundalini Energy arises (when awakened) from the root Chakra (Muladhara) to the crown Chakra (Sahasrara). In many Indian spiritual schools it’s thought that awakened and rising Kundalini Energy will lead to spiritual growth and finally spiritual enlightenment and bliss.
The Nadis play an important role in Yoga, as in many other Yogic practices, because they are intended to be open, and if blocked or obstructed, they need to be opened in order for vital life energy being able to flow uninhibited through the body. It’s thought that blocked Nadis and Chakras can cause illness or discomforts, thus consequently, Indic i.e. Ayurvedic healing has much to do with reopening or maintaining the Nadis and Chakras.
Like with the Sen Lines in Thai Massage and the Meridians in TCM, the Indian Nadis contain special points, that is, acupressure points or acupoints, which in Ayurveda and Yoga are called Marma points (by the way, in the Indian Tamil Nadu state they are called Varma points). Generally, 107 or 108 main Marma points are recognized, and they are manipulated (with acupressure, stretches, and Yogic exercises, and such) to cure specific physical, mental, emotional and spiritual illnesses or discomforts.