The sacred word designating this creative source is the sound OM, called pranava.
Pranava [Om] is the denoter of That [God].
The word expressive of ishvara is the mystic sound OM [OM is god’s name as well as form].
This sutra is saying that ‘God’ or the supreme consciousness is in the vibrations of the sound ‘AUM’. The sound is not just the name of the supreme consciousness, but is present in the vibratons of the sound itself when you say it. It conveys the exact nature of it.
Sri Swami Satchidananda quotes the Mandukya Upanishad which says that AUM contains seeds of all other sounds. He describes it as a hum, connected with prana (the basic vibration or vital life force), & says that when you meditate, you experience this hum.
This blog explains the visual OM symbol nicely:
The lower curve represents the Gross, Conscious, and Waking state level, called Vaishvanara.
The center curve represents the Subtle, Unconscious, and Dreaming level, called Taijasa.The upper curve represents the Causal, Subconscious, and Deep Sleep level, called Prajna.
The dot, point, or bindu represents the fourth state, the absolute consciousness, which encompasses, permeates, and is the other three, and is called Turiya.
The arc below the dot symbolizes the separateness of this fourth state, standing above, though ever remaining part of the other three.