maharshi:na:m bhrugur aham
gira:m asmye:kam aksharam |
yajna:na:m japayajno::smi !
sttha:vara:na:m hima:layaha || 25  - Bhagavad Gita,Chapter 10

Of the great seers I am Bhrigu; of words I am the one syllable 'AUM'; of sacrifices I am the Japa sacrifice; of things immovable, the Himalaya.  - Bhagavad Gita, Chapter 10, Verse 25

Krishna has clearly said the word 'aum' is he himself. This syllable has the power greater than all.

Meditating with this syllable, they say, purifies the kundalini chakras present in human body. There are seven kundalini chakras or energy centres in the spinal region of our body. One needs to activate those energy centres to attain the great spiritual enlightenment. This chakras are said to be the pathway of enlightened consciousness of human beings. Upon activating each one attains the higher state called 'Samadhi' ( fyi, samadhi in a single sentence is said, as a thoughtless moment but with complete consciousness of existence).

The second chakra named Ajna or third eye chakra which depicts the dual state of Shiva and represented by the syllable Aum.

This Ajna is the chakra of time, awareness and of light.

some analysis says about this chakra is-

"Ajna, is known as the third eye chakra and is linked to the pituitary gland which secretes hormones to communicate to the rest of the endocrine system and also connects to the central nervous system via the hypothalamus. According to Gary Osborn, the thalamus is thought to have a key role in the physical basis of consciousness and is the 'Bridal Chamber' mentioned in the Gnostic scriptures. Ajna's key issues involve balancing the higher and lower selves and trusting inner guidance. Ajna's inner aspect relates to the access of intuition. Mentally, Ajna deals with visual consciousness. Emotionally, Ajna deals with clarity on an intuitive level." (taken)

Thus, the bija(seed)-mantra 'aum' for this chakra is equally important as the chakra itself. Chanting this mantra for 7-10 minutes with the proper mudra can open your consciousness to the greater spiritual heights. 

Locations of the kundalini-chakras and the mudra for third-eye chakra respectively:

May light bless you all.