Three sounds, one syllable
Om (ॐ) is treated as the primordial sound from which all of creation arises — not a word with a dictionary meaning, but a sound considered to underlie all other sound. Written and chanted, it's actually built from three phonetic parts — A, U, M — plus a fourth, silent element some traditions add: the stillness after the sound fades.
What the Upanishads say it represents
The Mandukya Upanishad, entirely devoted to explaining this one syllable, maps its three audible parts onto the three ordinary states of consciousness: A to waking awareness, U to the dream state, and M to deep, dreamless sleep. The silence that follows is mapped onto a fourth state, turiya — pure awareness underlying the other three, sometimes translated simply as "the fourth." Other traditions map the same three sounds onto past, present and future, or onto Brahma, Vishnu and Shiva as creator, sustainer and dissolver.
Why it opens almost every mantra
Because Om is treated as the root sound beneath all others, chanting it before a mantra is less like a formal preamble and more like tuning an instrument before playing — it's meant to settle the mind and voice into the right register before the specific request or praise that follows. This is why you'll see it at the start of the Gayatri Mantra, the Mahamrityunjaya Mantra, and most traditional Sanskrit prayers on this site.
Chanting it on its own
Om is also chanted by itself, often in a single long, slow exhale, sometimes as the entire content of a meditation session. Many people find the physical vibration of the sound — felt in the chest and throat as much as heard — is part of its calming effect, independent of any philosophical interpretation.