Chakras are centres of energy which govern certain zones of the body; they convey energy between dimensions (or planes of existence) and are centres of subtle forces, the generation of prana (energy) and higher consciousness.
Ajna is located between the two eyebrows and is the source of two nervous flows, one through the eyes and the other through the mid-brain.
This chakra governs inner vision and the dynamic activities of will and knowledge. This “third eye” is associated in many cultures with light, inner knowledge, intuition and psychic abilities. Opening this chakra involves the integration of both the intellectual and emotional poles. When Kundalini reaches this chakra, consciousness transforms.
The goddess of Ajna is Hakini Shakti, with six faces and six arms. When she is red this indicates Kundalini knowledge is fully awakened; when white she represents a state of calm; when dark-blue she is about to proceed into a formless state. If seen as a mixture of white, red and black she is demonstrating a blending of the three gunas – sattva (harmonious consciousness), rajas (activity) and tamas (inertia).
Meditation on this chakra is said to bring visions of the highest truth and ultimately the wisdom of the Self, the highest knowledge. It is often referred to as the governing chakra for all of the others, and some yogis recommend concentration only on the ajna, or initially there, before awakening the energies of other chakras. It can affect the development of the qualities inherent in all of the preceding chakras, and enable an aspirant to reach a state of non-dual awareness. Complete mastery of the lower chakras is believed to be only possible once you have acquainted yourself with ajna.
Information source: Energies of Transformation, A Guide to the Kundalini Process, by Bonnie Greenwell PhD.