What it means
Om is called the pranava. It has three parts, A, U and M. The Mandukya Upanishad links them to waking, dreaming and deep sleep, and calls the silence left after the sound the fourth state, turiya.
Where it comes from
The Mandukya Upanishad has just twelve verses, and all of them are about Om. It opens Vedic recitation, begins a puja, starts a yoga class. It doesn't belong to any one deity or sect, which is probably why it has travelled so far.
How to chant it
Sit up straight, take a deep breath, and let out a long O-M as you exhale. Give A, U and M each their room, and when your lips close on the M, let the hum carry on. Then rest a few moments in the silence. That silence is part of it.
When: Early morning is best, and at the start and end of meditation.
Try it now
The japa counter opens with this mantra selected and keeps count for you.