Kundalini  is an ancient  Sanskrit word derived from the root  kunda-, which means a coiled-up serpent, and  -ini meaning: power.)   
The kundalini is an energy that sits dormant in the first chakra at the base of the spine. This energy is released during orgasm through the second chakra, as the body attempts to create a child. During life-threatening situations, the kundalini energy is also often released. This is how people suddenly have the superhuman strength to save themselves or a loved-one. Most of us have heard about the grandmother who had to lift a car to save her grandchild. Her kundalini was released, giving her the power to do what she needed to do. Long ago, some yogis discovered that the kundalini can be used to move very quickly into higher states of consciousness. By meditating on the chakras, the kundalini is activated and begins to rise.

The kundalini is coiled up like a very tight spring, and when released there is sometimes an explosion of power. Properly directed, the kundalini will travel up a tube in the energy body that lies close to the spine. Without proper direction, the release of kundalini can be very uncomfortable and possibly damaging. (This is why it is very important to study kundalini yoga with an experienced guide.)

On it's way up, the kundalini both cleans and strengthens the energy body and chakras. You may feel a warm sensation moving up  your back.   If your head feels like it's burning,   you may be pushing too hard, too fast--be sure to consult with your teacher.

The caduceus symbol of coiling snakes
is thought to be an ancient symbolic
representation of Kundalini physiolog
You could think of kundalini yoga as a way of tapping into a natural storage of power that we all have to fuel your practice. Like the gasoline you put in your car, kundalini by itself is relatively harmless but when combined with will power, it can become volatile if not handled with respect

This is the dominant spiritual energy in all human beings and it is destined to be awakened. After awakening, this energy is the dynamic force behind meditation.
It is an elemental, creative force of the astral body which, like a serpent, rests coiled at the base of the spine. The male and female forces are represented in the balanced Ida and Pingala subtle channels. Everyone uses Kundalini power to think with and to maintain consciousness, but it very seldom rises up the central spinal channel of Sushumna beyond the first center. Various disciplines areused to  arouse the "sleeping serpent" to ascend to the higher centers.