According to the Mandukya Upanishad, the syllables of AUM represent the following realms of experience:
A - jagrat (waking state /conscious state)
U - swapna (dreaming state/ subconscious state)
M - shushupti (deep sleep state / unconscious state)
According to yoga the human body and mind are made up of three gunas, which are principles or 'tendencies' of Prakṛti (universal nature). The three syllables of AUM symbolise the three gunas as:
A - rajas - passion and dynamism,
U - sattwa - harmony, clarity and light,
M - tamas - ignorance, darkness and inertia.
After chanting AUM there is the silence which symbolises the transcendence of all the three gunas – trigunatita - pure consciousness.
The syllables of AUM also symbolise the three principles of existence:
A - Brahma, U - Vishnu, M – Shiva,
creative, sustaining and destructive principle.
After chanting AUM the silence that is present symbolically represents the underlying reality which underlies and is beyond that which is created, sustained and destroyed.
According to the Mandukya Upanishad the transcendental state is called Turya (Sanskrit meaning "4th") – that which is beyond and yet encompasses the three states symbolised by A, U and M.
The circle (or dot) symbolises turya, the underlying reality. Turya includes jagrat – the conscious waking state (A), swapna - the dream or subconscious state (U), shushupti - the causal unconscious state (M) and it is beyond them. The state of turya can only be comprehended when the mind dissolves into the consciousness that is beyond the three manifest states of the universe.