Pranavam [OM] is having the the combinations of Aa, Uu and Ma. The Pranavam is entirely submerged in these 3 letters.
Aa, Uu and Ma is AUM [OM]. Everybody knows it. But how paramatma jeevatma and merging of jeevatma with paramatma is depicted in AUM?
How does this Aa, Uu and Ma are correlated with respect to Paramatma and Jeevatma?
As per Mandukya Upanishad:
That same Self, from the point of view of the syllable, is Om, and viewed from the stand point of the letters, the quarters are the letters, and the letters are the quarters. The letters are a, u and m.
Vaisvanara seated in the waking state is the first letter a, owing to its all-pervasiveness or being the first. He who knows thus verily accomplishes all longings and becomes the first.
Taijasa seated in the dream is u, the second letter (of Om), owing to the similarity of excellence or intermediate position. He who knows thus verily advances the bounds of his knowledge and becomes equal (to all) and none who is not a knower of Brahman is born in his family.
Prajna seated in the state of deep sleep is m, the third letter (of Om), because of his being the measure or the entity wherein all become absorbed. He who knows thus measures all this and absorbs all.
That which is without letters (parts) is the Fourth, beyond apprehension through ordinary means, the cessation of the phenomenal world, the auspicious and the non-dual. Thus Om is certainly the Self. He who knows thus enters the Self by the Self.
As per AtharvaSikha Upanishad:
The sage Atharva replied to them,” Mainly meditation has to be done on the single letter Om. It itself is the mantra for meditation. The four legs of that mantra are the four devas and the four Vedas. The letter has to be recognized as the Para Brahman (Ultimate reality) and meditated upon. 1.2
The first syllable Aa, indicates earth, the Rig Veda along with its holy chants, Brahma the creator, Ashta Vasus among devas, “Gayathry” among meters, and Garhapthya (the fire of the household) among fires. 1.3
The second syllable Uu denotes the ether, the Yajur Veda, Rudra who is the God of destruction, the eleven Rudras among devas, “Trishtup” among meters and dakshinagni (fire of the south - funeral pyre) among the fires. 1.4
The third syllable Ma indicates the heavens, Sama Veda with its musical sounds of Sama, Vishnu who looks after the worlds, the twelve Adithyas (suns) among devas, “jagathichanda” among meters and Ahavagni (the fire used in fire sacrifice) among the fires. 1.5
That half fourth syllable which is the hidden Ma, is the magical chants of Atharva Veda, Samvarthaka (the fire of exchange) fire among fires, marud Ganas among devas. It is the self glittering Brahman which shines alone and sees everything. 1.6
Apart from the explanations given in previous answers i would like to add the following:
The usual description of the A,the U and the M in AUM is the following:
Veda Vyasa further explained that the Word ‘A’ meant Formation, ‘U’meant Protection and ‘M’ meant Demolition, reflecting the Three Characteristics of Satvika Guna, Rajasika Guna and Tamasika Guna. Most importantly, Veda Vyasa stated that the emphatic stress while pronouncing the last word or the Half Word ‘M’ of ‘OM’ stood for Maha Maya Bhagavati Herself. The speciality of the terminating half letter ‘M’was that it was not pronounced as a distinct and separate word, signifying thereby that the Half Word was ‘Maya’ or ‘Maheswari’ which had no Entity yet the ‘Most Distinctive’! [ ‘Vyakta Avyakta Swarupini’ or ‘Saguna-Nirguna Embodiment]
Shiva shadakshara Stotram starts with the verse "Omkaram bindu samyuktam nityam dhayanti yoginah.."-Meaning-Shiva,Who is OM with the bindu & who is always meditated upon by Yogis..".
So, its natural to ask for a description of AUM related only to Shiva.
Linga Purana infact gives exactly such an explanation which corelates the various alphabets(including those of AUM) to different body parts of Shiva.
Maharshi Lomarshana, the disciple of Veda Vyasa, while addressing the Sages at Naimisha Forest described that at the Maha Pralaya or the termination of the Universe after the Maha Yuga, the Supreme Bhagavan created Vishnu with the aid of Satwa Guna who was floating on Maha Jala or the Unending Water Sheet and from his navel appeared a lotus with Brahma as materialised with Rajo Guna seated thereon; the latter questioned Vishnu about his antecedents and consequent on mutual arguments about their Supremacy there was a fight; at that very time there appeared a Colossal Agni Linga emitting enormous radiance and heat..........
....As neither the Hamasa Rupa Brahma who flew up and up for thousand years nor the Varaha Rupa Narayana who slided down as much time could discover the bottom or the top and returned to the starting point by another thousand years; both of them reached there totally fatigued. Then there was a reverberating Sound of AUM:
Tadaa sama bhavattatra naadou vai Shabda lakshanah,
Omomiti Surasreshtthaah Samyuktah plutalakshanah/
Kimidam twiti samchitya mayaa tishthanmahaaswanam,
Lingasya Dakshiney bhaagey tadaapashyat Sanaatanam/
Adya varnamakaaram chouttareytath,
Makaaramadhya taschaiva naadaantam tasyachomiti/
Suryamandalavadrustwaa varnamaadyam tu Dakshiney,
Uttarey Paavaka prakhya mukaaram Purusharshabhah/
Sheetaamshu Mandalaprakhyam Makaaram madhyamam tathaa,
Tasyopari tadaapashyat- kshudra sphatikavat Prabhum/
Tureeyaatimamrutam Nishkalam Nirupaplavam,
Nirdwandam Kevalam Shunyam Bhaahyaantara varjitam/
Sa baahyaabhyantaram chaiva Sabaahya abhyantarasthitam,
Adi madhyantarahita maanandasyaapi kaaranam/
Maatraastistwardha maatram naadaakhyam Brahmasangnitam,
Rukyajurssaama Vedaa vai Maatraa rupena Maadhavah/
Vedashabde -bhya yevesham Vishwaatmaanamachintayat,
Tadaabhavadrushirveda Rusheh saaratam shubham/
Teynaiva Rishinaa Vishnurjnaatawaan Parameswaram/
( As the sound of AUM emerged from the right side of the Maha Linga, there appeared Sanatana Bhagavan; from Him, the sound of ‘A kaara’ came, followed by ‘U Kaara’ and ‘Ma kaara’ and the sound in between was the ‘Naada Swarupa’; the extraordinary radiance of Surya came from the Northern direction as the pious form of U kara or of Pavaka; Makara was of Chandra Mandala and above it was the Suddha Sphatika Rupa or Pure Crystal Form representing Maha Deva in Tureeya Awastha or the Prime and Pristine Form. This Form was Unique, devoid of Duality, of Sunya or Nothingness; of Purity and Immunity and of no beginning-middle- termiation. Indeed that was the Spring of Ananda or Sheer Joy as also the synthesis of Tri Vedas represented by Tri Murtis and the sure way leading to Maha Deva!)
> Maharshis who knew Vedas and the Omkara Swarupa of Maha Deva analysed the Alphabets in Sanskrit and explained as follows: ‘A’ kaara’ constituted His broad forehead; ‘E’ kaara was the left Eye; ‘U’kaara his Southern Ear; ‘Aa’ kaara his left ear; ‘Ru’ kara his right Kapola; ‘Ruukaara’ the left kapola; ‘lu lu’is the partition of his nasal divisions; ‘ea’- kaaras his big lips; ‘oau’ kaara his teeth; the consonants ‘Ka-kha-ga- gha-jnaa’ constituted his five right hands while the consonants ‘cha-ccha- ja-jja-na’ were his left five hands; other two sets of consonants viz. ‘ta-tha-da-dha-na’ and ‘pa-pha-ba-bha-ma’ were his right and left five feet respectively; Shiva’s ‘udara’ or belly represented ‘paa’ kaara; ‘pha’ kara his right side of his body; ‘Va’ kara his left side; the letter ‘Vaa’ and ‘bha’ kaaras represented his shoulders; the letter ‘Ma’ kaara his ‘Uridaya’ or heart; ‘Ya- ra-la-va-sa-sha’ represented Shiva’s body-dhatus; ‘ha’ kara represented His Atma and ‘Kha’ kaara denoted Shiva’s anger.
OM represents the Self which is the Supreme Nondual Reality. The Self is known in four states, namely, the waking state, the dream state, the deep-sleep state, and the fourth state called the turiya. All these states are represented in the three sounds of OM (i.e., A, U, M), and the silence that follows and surrounds the syllable.
The sound A represents the waking state he sound U represents the dream state, and the sound M represents the deep sleep state.
The waking state is superimposed on the A sound because it is the first of the three states of consciousness, and so is the sound A the very first of the letters of the alphabet – in all languages. The dream is but a view within the mind of the impressions that had reflected on the surface of the mental lake during the waking state. Besides, the dream state occurs between the waking and the deep-sleep state, and comes second among the three states of consciousness. And so, U, being next to A in order of sounds, and also since it is between A and M, is treated as representing the dream state. On the M sound of OM is superimposed the deep-sleep state. The comparison between the last sound of the OM and sleep lies in that it is the closing sound of the syllable, just as deep sleep is the final stage of the mind in rest.
A short pregnant silence is inevitable between two successive OMs. On this silence is superimposed the idea of the fourth state, known as turiya. This is the state of Perfect Bliss when the individual Self recognises the identity with the Supreme.
