In nutshell the Mool mantra is summed up as follows.
ਪੁਰਖੁ ਨਿਰਭਉ ਨਿਰਵੈਰੁ ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੈਭੰ ਗੁਰ ਪ੍ਰਸਾਦਿ ॥
Ik oankar satinamu karta purakhu nirbhau nirvairu akal murati ajuni saibhan gurprasadi
translates to One Supreme Being, Truth is His name; the Creator Primal Being; Without fear and Without Enmity, the Timeless Verity, Un-incarnated and Self-Existent, known through His grace.[(GG. Pg 1)]
Oankar is a variation of the mystic monosyllable Om (also known as anahata nada, the unstruck sound) first set forth in the Upanishads as the transcendent object of profound religious meditation.
Guru Nanak prefixed the numeral one
(ik) to it making it "Ik Oankar" or "Ekankar" to stress GOD's oneness. GOD is named and known only through GOD's Own immanent nature.
{for detailed discussion refer:
http://www.sikhphilosophy.net/essays-on-sikhism/15076-god-in-sikhism.html]
As said Guru Tegh Bahadur, Nanak IX, "He has himself spread out His Own "maya" (worldly illusion) which He Himself oversees; many different forms He assumes in many colours, yet he stays independent of all" (GG, 537).
God is Karta Purakh: the Creator-Being. He created the spatial-temporal universe not from some pre-existing physical element, but from His own Self. Universe is His own emanation. It is not "maya" or illusion but is real (sat) because, as say Guru Arjan, “True is He and true is His creation [because] all has emanated from God Himself” (GG 294). But God is not identical with the universe. The latter exists and is contained in Him and not vice versa. God is immanent in the created world, but is not limited by it. “Many times He expands Himself into such worlds but He ever remains the same One Ekankar" (GG, 276). Even at one time "there are hundreds of thousands of skies and nether regions" (GG, 5). Included in Sach Khand (Realm of Truth), the figurative abode of God, there are countless regions and universes" (GG, 8). Creation is "His sport which He Himself witnesses, and when He rolls up the sport, He is His sole Self again" (GG, 292). He Himself is the Creator, Sustainer and the Destroyer.
What is the Creator's purpose in creating the universe? It is not for man to enquire or judge the purpose of His Creator. To quote Guru Arjan again, "The created cannot have a measure of the Creator; what He wills, O Nanak, happens" (GG, 285). For the Sikhs, the Creation is His pleasure and play "When the showman beat His drum, the whole creation came out to witness the show; and when He puts aside his disguise, He rejoices in His original solitude" (GG, 174, 291, 655, 736).
That God is
"nirbhau" (without fear) and "nirvair" (without rancour or enemy) is obvious enough as He has no "sarik" (rival). But the terms have other connotations, too. Nirbhau not only indicates fearlessness but also the absence of fearfulness. It also implies sovereignty and unquestioned exercise of Will. Similarly, nirvair implies, besides absence of enmity, the positive attributes of compassion and impartiality. Together the two terms mean that God loves His handiwork and is the Dispenser of impartial justice, dharam-niau. Guru Ram Das, Nanak IV, says: "Why should we be afraid, with the True One being the judge. True is the True One's justice" (GG, 84).
God is Akal Murat, the Eternal Being. The timelessness involved in the negative epithet akal has made it popular in Sikh tradition as one of the names of God, the Timeless One, as in Akal Purakh or in the slogan Sat Sri Akal (Satya Sri Akal). One of the most sacred shrines of the Sikhs is the Akal Takhat, the Eternal Throne, at Amritsar. Murat here does not mean form, figure, image or idol. Sikhism expressly forbids idolatry or image-worship in any form. God is called "Nirankar", the Formless One, although it is true that all forms are the manifestations of Nirankar. Bhai Gurdas, the earliest expounder and the copyist of the original recension of Guru Granth Sahib, says: "Nirankar akaru hari joti sarup anup dikhaia (The Formless One having created form manifested His wondrous refulgence" (Varan, XII. 17). Murat in the Mool Mantra, therefore, signifies verity or manifestation of the Timeless and Formless One.
God is Ajuni, Un-incarnated, and Saibhan (Sanskrit svayambhu), Self-existent. The Primal Creator Himself had no creator. He simply is, has ever been and shall ever be by Himself. Ajuni also affirms the Sikh rejection of the theory of divine incarnation. Guru Arjan says: "Man misdirected by false belief indulges in falsehood; God is free from birth and death. . . May that mouth be scorched which says that God is incarnated" (GG, 1136).
The
Mool Mantar ends with gurprasadi, meaning thereby that realization of God comes through Guru's grace. "Guru" in Sikh theology appears in three different but allied connotations, viz. God, the ten Sikh Gurus, the enlightened ones and enlighteners, and the gur-shabad or Guru's utterances as preserved in the Guru Granth Sahib. Of God's grace, Gurus' instruction and guidance and the scriptural Shabad (Sanskrit, sabda, literally 'Word'), the first is the most important, because, as nothing happens without God's will or pleasure, His grace is essential to making a person inclined towards a desire and search for union with Him.
Extracted from adopted :
http://www.sikhphilosophy.net/essays-on-sikhism/15076-god-in-sikhism.html]