What is Kundalini, you asked me?
It's a tough concept to put into words. In the physical world, our bodies run on various fuels such as food, liquids, and oxygen. But the physical body is only the most superficial manifestation of what we really are. There is also a subtle physical body, also known as the aura, which as we learn to meditate we gain more direct awareness of. When we discussed chakras a few weeks back, the chakras we talked about are in the subtle physical, not the physical, body.
At the subtle level, Kundalini is the fuel which keeps our subtle bodies running. Just as we need food and water to survive on the gross physical level, so we also need Kundalini to survive at the subtle level. So everyone who is alive has a certain amount of Kundalini flowing through their subtle bodies.
However, in this world at this time, most people are operating at a starvation level with regard to Kundalini energy. They have just barely enough flowing to keep alive, nothing more. The Kundalini energy resides at the base of the spine, in the first chakra, and in the process of living some of it will be brought up to higher level chakras to fuel our lives. When we meditate, we are trying to do this on a conscious level so that instead of getting just a minimal level of Kundalini, we feel its power flowing through us.
How does it feel when the Kundalini is flowing? It's a tough thing to describe, but you just feel better about life. You have more energy; you are happier, you get sick less often, and you just tend to see the bright and beautiful side of everything. It isn't some warm fuzzy feeling, but rather you just feel life at a more intense level instead of being largely numb to it.
The process of meditation is the process by which you bring Kundalini energy up from the lowest chakra to the higher level chakras, and experience the more refined states of mind associated with those chakras, ultimately bringing the Kundalini up to the crown chakra and attaining Enlightenment.