Kundalini is an energy that exists in everyone's body, usually in a dormant state. This means that most people never feel it and never know it is there. But in a very few people, perhaps one in one thousand, this energy becomes aroused, activated. This can be a happy event or it can be scary and disruptive, depending on whether you aroused your kundalini on purpose or by accident. The Sanskrit word, kundalini, means coiled, like a snake. Kundalini energy is not recognized by medical science, and is often little understood even among teachers of Yoga and meditative traditions. It is, however, mentioned extensively in the literature of Yoga and Tantra (both Buddhist and Hindu).
Here is the story of kundalini as found in the Hatha Yoga literature , and as I have experienced it: Kundalini can be "awakened" or "aroused" from its "slumber" at the base of your spine by intense meditation or intense breath control practices. Hatha Yoga and Tantra Yoga, in their traditional forms, are designed to arouse kundalini so that the practitioner can use the tremendous energy thus released to increase the potency of his or her meditation and other spiritual practices. The Yoga literature cautions the Yoga practitioner, however, that he or she must undergo extensive preparations before attempting to arouse his or her kundalini. Otherwise it can cause trouble.
The yogi or yogini who successfully and safely arouses their kundalini gains a great ally on their spiritual journey. That ally is an energy, a vitality, that smooths the path, that makes the steep slopes easier to climb, and that acts like a sword to cut through inertia and conditioning. In short, Kundalini can change you quickly. It can advance your spiritual aspirations. Moreover, according to authors of some of the Tantras (ancient writings), aroused kundalini energy is essential to a yogin (practitioner of yoga, male or female) attaining complete liberation in this modern distracted age (called Kali Yuga).