When a thought, oriented towards God, becomes effective, the light of the being is expressed. It is That which reveals the splendor inside the word, the secret of the thought, and which conserves the rhythm. Man repeats the rhythm of the Eternal! It is God himself which illuminates. The Scriptures say: “This is what is known by the Vedas. The mantra of divine consciousness brings the light of revelation.”
The mantra of divine power brings the will to carry out.
The mantra of bliss, ananda, brings the fulfilment of the spiritual joy of existence.
All words and thoughts emanate from the flow of vibrations contained in the sound Aum, which is Brahman, the Eternal.
Behind the manifestations of forms and perceivable objects, behind the continual game played out in our self, where forms of objects are figures, behind the manifestation of supra-consciousness and of infinite power, there is Aum…the ultimate source of seeds and matrices, of things and ideas, of names and forms. Aum is itself none other than the supreme Unity, intangible and original, existing by itself, beyond all manifestations. The Vedas were considered as mantras, and even several parts of the Veda are called mantras. This means that they must be repeated, insofar as the mantra does not depend so much on its meaning, but rather from the sound produced by repeating it.
The idea that the sound Aum expresses the supreme Brahman is also found in the Mandukya Upanishad, whose rather short text received an important karika(commentary) written by the philosopher Gaudapada, and on which Sri Sahnkacharya composed some well-known commentaries.
This Upanishad explains how Aum represents Everything, in other words how the repetition of Aum is the most important mantra.
The Katha Upanishad, the Mundaka Upanishad, and other texts recommend to concentrate on Aum. Sri Aurobindo explained that Aum represents all of the Vedas, all of creation, and God himself. Having this awareness, knowing that Aum represents God, and meditating on Aum, enables one to attain a state of supra-consciousness, of direct relation and communion with the Lord.