It is the primordial sound, the original sound that contains in it the universal vibration. The repetition of “OM” allows us to come into harmony with the universal consciousness. the « Pranava Śábda » pronounced AOUM… This is the most used mantra.
“The self is of the nature of the syllable Om…
Meditate on Om, it is meditated on the self”-
Māṇḍūkya who 1.8.12, 2.2.3)