(I met a wise man years ago, namely Prof / Pandit Saraswati Prasad Ghimire. He had this to tell me. Sri Ghimire is no longer with us.)
AUM has infinite meanings. It is the infinite itself. There are infinite ways or paths to realise infinity. One of them is realisation of AUM thru the way of the Upanishads.
Upanishad is a treatise based on Vedas, the oldest in Hindu teachings. They are said to be composed by enlightened seers and sages. Vedas deal with existence of man and universe. Upanishads follow them in theme and variations.The meaning of AUM is explored in an analytical/experiential way with extraordinary understanding and clarity in Mandukya Upanishad. Later commented by none other than the first Shankaracharya, it takes us to the Absolute, i.e. understanding our real nature.
TASYO PAVYAKHYA-NAM BHOOTAM BHAVAD-BHAVISHYAD
ITI SARVAMOM-KARA EVA
OM-KARA EVA (Mandukya Upanishad 1)
(AUM, the letter is everything. Thus it is explained. The past, the present, the future and whatever else beyond the three times is also AUM.)
To explain this, first let us understand that to realise the Absolute, there are ways of Negation and Affirmation. Neti, Neti (Not this, not that) of the teachings is the way of Negation. In short, it negates everything that is not Absolute. Hence Absolute is the last remainder ( speaking under the limits of language). Another example of the way of Negation is Buddhist Master Nagarjuna's teachings. In his dialectics, he also negates all the possibilities of vocabulary and conceptions. (na sat, na asat …….)
In the way of Affirmation, AUM is the explanation or expression of the three dimensions of time, viz past, present and future and the substance or content inside these. It is what we see, hear, smell, taste and feel. In reality, everything is AUM. Smell is not. Hear is not. Only AUM is. All what was/happened (past), what is/happening (present) and what will be/will happen(future) are AUM. Time is dependent on it. Time gets its substance or identity from it. All our concepts, percepts, whatever you name or is formed are nothing but expressions of AUM. Again, whatever else beyond the three times and what is not explained or expressed are also AUM. You cant even imagine a thing that is not dependent on it, that is not it in its essential nature. Nothing is different from it. What we do not see, hear, smell, taste and feel are also AUM. It is beyond time or timeless. There is nothing which is not in its domain or range. AUM is infinity. To be a bit technical, everything from minus infinity ( infinitely small) to plus infinity (infinitely big) is AUM. It is every time, every space. It is the three times and beyond the three times. You are AUM too. AUM is you. It is ownness, nijata in Sanskrit, aafai in Nepali, meaning self. But this self is neither bound nor ego. One needs to imbibe this. One needs to taste, chew and digest, to paraphrase Bacon. Realise and experience this. Mere repetitions or intellectual understanding wont do. What is needed is first hand knowledge, direct experience. Practise this regularly from your heart, not head. Through the grace of AUM, may all be happy.