Acharya Prashant explains that when you say 'A-U-M', you have described the mind in its totality, as the mind has no state other than these three. By talking about these three, you have circumscribed the mind. These are the three states of consciousness. The first one, 'A', is the waking state ('Jagrat Avastha'). The Rishi realizes with such intensity that he gives a separate, dedicated name for your existence in the waking state, calling it 'Vaishvanar'. This is because you are not the same person in the waking state as you would be in the dream or deep sleep states. Vaishvanar is the one who is conscious of the world ('Vishwa'), who lives in and experiences the world through the sense organs, which are the gates in the body. The moment you fall asleep, you are a totally different person, so you must be given a different name. This is the 'U' state, or the dream state ('Svapna Avastha'), and your name is now 'Taijas'. You are in a different world, and your personality has entirely changed from what it was in the waking state. Then there is the 'M' state, which is the deep, dreamless sleep state ('Sushupti'). In this state, all the gross remnants of personality dissolve. All that remains is a stress-less, tension-less relaxation, which can be called bliss. All the identities you carry, whether in dream or waking, are a kind of tension. In dreamless sleep, the greatest relief is that you have no name, no past, no future, and no identity; there is just a void. However, you have not totally gone away; the seed or germ of your existence is still there in its most rudimentary, primordial form. Because this seed form is present, manifestation can take place again when you are shaken out of your sleep. Following these three states is the dissolution, the silence, which is the fourth quarter ('Turiya'). This is the substratum of the mind, the Self (Atma or Brahman), which exists irrespective of the mind. It is not really the fourth state, but the fundamental one. The way 'Om' is pronounced, tapering and fading away into silence, is a musical representation of this process. 'Om' is a remembrance that firstly, everything is mind, and secondly, there is peace beyond the mind. It is meant to remind you that you too have to end and fade away.