Prana is the name that the sages gave to the life force that becomes the trees, the radiant currents of sunlight, the negative ions in the atmosphere, and the nourishment in water. Before becoming this world of matter, the creative energy of this universe (Shakti) evolves into prana, a form of energy that is slightly grosses than pure Consciousness and that links the relatively thick and solid physical universe with its subtle essence.
When the prana slows down (a state that yogis try to induce through practicing pranayama), the mind quiets down in response. That is why following the breath in meditation is so helpful in quieting the mind.
The sensations of prana are threads that connect us to Shakti. Prana is the vehicle we ride as the awakened Shakti gently leads us through the layers of our subtle being.
Prana is a psychic substance which inhabits our subtle or astral body. Though it does not reside in the physical body, it is the energy which activates this body, vitalizing and coordinating all the countless functions we perform in life, such as walking, talking, eating, sleeping, working, playing, etc. Prana also powers the mind. It activates our physical senses, conveys their impressions to the brain, and it even processes those impressions; moreover it provides the energy which runs our ego, intellect and will, and every thought we generate is made of it.
Prana puts our body together before birth, holds it together and gives it life, then departs to let it decompose in death.