A Nadi is a vessel which carries Prana. Our Vital Body or Pranamaya Kosha contains a network of vessels, or a kind of channel like nerves in a grid. This network carries subtle energy from the Sun. There is a complex network of Nadis.
THE THREE MAIN NADIS
Yoga Shastras mention that there are 72,000 Nadis. The three major Nadis are Ida, Pingala and Sushumna. Whenever a lot of energy flows in a particular Nadi, that Nadi will be active.
Ida or Chandra Nadi is on the left side and Pingala (Surya Nadi) on the right side of our spinal chord. And Sushumna Nadi is in the middle.
These three Nadis start from the base of our spine. The Ida and Pingala pass through the Chakras and end at the Agna Chakra. The Sushumna Nadi does not pass through any of the Chakras, but it reaches the Agna Chakra. All three join at the Agna Chakra, but the Sushumna continues and reaches the Sahasrara Chakra. The Sushumna is the most important Nadi because it is the only Nadi which connects the Mooladhara to the Sahasrara.
Surya Nadi carries the Prana which comes directly from the Sun. Chandra Nadi carries the Prana which comes indirectly, that is the Prana reflected by the moon. Planets also reflect Prana, which we receive. Many networks in our own Nadi network that carry the Prana are directed back to us by different planets.
The Nadis will not be active all the time. From the sunrise, the three Nadis - Chandra, Surya and Sushumna Nadis will be active one by one for 24 minutes each. The cycle repeats.
Like everything else in Creation, the Pranas flowing in them have their own intelligence. So, whenever a Nadi is active, it brings a certain influence.
The activation of the Chandra Nadi helps one to be very efficient in Material activity. That of Surya Nadi helps to be efficient in Spiritual activity.
When Sushumna Nadi is active, it helps us to experience Samadhi, to transcend many petty things in this life. It takes us to a higher consciousness. Whenever this Nadi is active, we can never think in a small way. We will be in a kind of expanded awareness. All noble thoughts will be with us at that time. If we meditate, when the Sushumna is active, we can experience Samadhi easily.
When we meditate and advance in Meditations, all the other Nadis will not be that active and only the Sushumna will be very active. The same Nadis will not be active in everyone at the same time. It varies from an individual to another individual. By practice, we can know which Nadi is active in our system. We can also know it by intuition.
Planets influence our thinking, emoting and our actual living. A combination of a set of planets will have a particular influence on us and we act accordingly. This is how we acquire Karmas. We can go beyond all these influences, make all these influences nil by meditating.
Spiritual energy, the undiluted and unconverted, is stored in the Kundalini. This energy is circulated in the Nadis. Pure Kundalini rises through the Sushumna Nadi without touching any Chakra. The not-so-pure energies pass through the Ida and Pingala Nadis.