Nadis are the subtle, psychic channels in the subtle body.
In the subtle body, there are fourteen principle nadis out of which three nadis are most important - Ida, Pingala, and Sushmana.
These are the channels or the pathways through which prana (life energy) flows. Like chakras, these nadis aren't physically visible inside the body.
Sushmana is the central psychic channel.
Through the Suhsmana Nadi, the Kundalini energy moves up from the Muladhara chakra, energizing the subsequent Chakras.
Sushmana is associated with river Saraswati.
Ida and Pingala Nadis meet Sushmana Nadi at the Ajna Chakra or the third eye Chakra, thus ending the duality, thereby bringing a balance between the masculine and feminine at an awakened state.