The first chakra is known as Muladhara Chakra and is situated at the base of the spine. It is often called Root Chakra because the word Muladhara means "Root Place" in Sanskrit. In Kundalini Yoga the latest Kundalini energy is considered to begin at this chakra. This chakra is associated with the element of earth and our basic needs, such as eating. It is all about moving down in order to come up. If you have an under active root chakra you may feel rootless, blown about in the wind. An over active root chakra may lead to over or under eating. A balanced root chakra enables you to feel secure in yourself and grounded. Poses that stretch and strengthen the legs are asanas for Muladhara Chakra. Also recommended are mula bandha (root lock, an advanced yoga technique) and being in nature.
Much of this information comes from the work of Anodea Judith, whose works are widely available.