The syllable AUM or
OM, with which every recital of the Vedic chants begins, is represented in the first stanza of Chandogya Upanishad as the symbol of the Supreme and therefore the means of the meditation of the Supreme.
(Source: The Principal Upanishads - Chandogya Upanishad first stanza interpretation)
In meditation, the soul is furnished with a symbol on which we fix our gaze, on which we concentrate all our imagination and reasoning. When meditation reaches its end, when there is no distraction or disquiet, when there is calm repose, sweet tranquility, there is the vision. We start with prayer, we pass on to meditation. When the discursive acts cease, we have contemplation.
The Chandogya Upanishad opens with the instruction to concentrate on the syllable aum, to draw our thoughts away from all other subjects, to develop ‘ekagrata’ or one-pointedness.
Symbol cannot be taken as final. It has a number of aspects. When it is transposed into the words of ordinary language it becomes dim and rigid. We then tend to confine the meaning within narrow dogmatic frames. Even though the syllable AUM, like all symbols covers the reality as by a veil, to those who know how to look, the veil becomes transparent.