Mool Mantra is the opening verse of Sri Guru Granth Sahib Ji. The author of the verse is Guru Nanak Dev Ji. Mool Mantra as written below is the description of the qualities of ੴ – the Universal energy, what people call as God.
ੴ ਸਤਿ ਨਾਮੁ ਕਰਤਾ ਪੁਰਖੁ ਨਿਰਭਉ ਨਿਰਵੈਰੁ ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੈਭੰ ਗੁਰ ਪ੍ਰਸਾਦਿ ॥
The word Mool (Mul) means root, primordial, basis or foundation. Mantra (mantar) means a sacred formula or a combination of sacred words. So the mool mantar is the root mantra that forms the basis of the holy granth.
ORIGIN OF MOOL MANTRA
Guru Nanak Dev Ji revealed the mool mantar for the first time at Gurudwara Sant Ghat Sahib which is situated in Kapurthala district of Punjab. Gurudwara Sant Ghat Sahib is the place located on the bank of river Bein (Vein), where Guru Nanak dev Ji re-emerged after 3 days of his disappearance at the site of Gurudwara Shri Ber Sahib.

MEANING OF MOOL MANTRA (MUL MANTAR)
What Guru Nanak Ji is trying to say in these few words are the different qualities or virtues of ੴ (God). The meaning of this verse is philosphically dense, extremely difficult to put in few words as it encloses the essence of the spiritual message of Guru Granth Sahib Ji (1430 Ang).
Let us try to have a look at what Guru Nanak Sahib Ji is trying to tell us, word by word.
ੴ – refers to One Universal Being that is omnipresent in all living as well as non-living beings. By adding the numerical ੧ in the beginning, Guru Nanak Dev Ji tries to tell us that there is only one without the second. Oankaar – means that the One supreme reality (what we call as God or Bhagwan) constantly manifests itself throughout the entire creation in various forms. Oankaar is present everywhere.
ਸਤਿ ਨਾਮੁ (Satt Naam) – Satt means permanent, which remains forever, that does not changes, the Ultimate reality. Naam here refers to the names given by the devotees to that Universal Being (ੴ ) and the qualities of that Universal Being. So Satt Naam means that the Univeral Being is the ultimate truth that never changes and the names or the qualities that are used to describe that Universal Being(ੴ ) is true. Everything else is false and temporary. Only the personality of the Universal Being(ੴ ) is true.
Karta Purakh – Karta translates to the doer or the creator. Creator again is a word used to describe ੴ (Oankaar or Univeral Being) . Other meaning of Karta is “the act of doing”. Purakh refers to the ੴ (universal being) that is submerged in its own creation. So Karta Purakh is a quality of ੴ , that means that the universal being is doing everything. Since ੴ is present everywhere in the creation, we all are made of ੴ , so whatever happens is due to that One.
Nirbhau Nirvair – Nirbhau means without fear and Nirvair means without hatred. Nirbhau Nirvair being the quality of that universal being (ੴ ), here states that God is not afraid of anyone and it does not holds any hatred or ill-will against anybody.
Akaal Moorat – Akaalmoorat means beyond time, death and form. The word Akaalmoorat refers to the Universal being (ੴ ) as an eternal formless insdestructible entity.
Ajooni – Ajooni means beyond birth or existence. The word Ajooni here is used to describe the quality of Universal being (ੴ ), that the supreme being is beyond incarnation.
Saibhang – Saibhang means self-existing. The word Saibhang is derived from sanskrit words – Swayam (self) + Bhoo (existent). Saibhang is the power of the Universal being (ੴ ), which means that it is self creating, it exists by itself without any cause.
Gurparsad – Guru refers to the master who is the dispeller of ignorance. Parsad refers to the master’s favour or grace. Here Gurparsad refers to that the Universal being (ੴ ) can be only realised by the true Guru’s grace.
Jap – To meditate or remember that Universal being (ੴ ). The mind has to meditate or remember that One Reality(ੴ ) every moment of our lifetime.
Aad Sach Jugaad Sach – Aad Sach means true in the primal beginning & Jugaad Sach means true throughout the ages. Meditate on that universal being who was true in the beginning & still is throughout the ages.
Hai Bhi Sach, Nanak Hosee Bhi Sach – Hai Bhi Sach means True in this instant. Nanak Hosee Bhi Sach means O Nanak, forever and ever true.
Mool Mantra – Sometimes Mool Mantra is also spelt as Mool Mantar or Mul Mantra.