Guru - The Divine Light
GU- means darkness and
RU- means Light.
In Sikhism the word 'Guru' is, thus, defined as the Light that dispels all darkness, and that is called JOT (Divine Light). Guru Nanak was, therefore, the EMBODIMENT of Divine Light:
'Gur Nanak Dev Govind roop.' 'Guru Nanak is embodiment of the Light of God.' Ang 1192 SGGSJ
The Guru in Sikhism is a perfect Prophet or Messenger of God in whom the Light of God shines fully. The True Guru is in union with Divine Lord of all. Thus he ushers the devotees, the seekers of Truth into a spiritual path of truth. Through the Guru the Glory of the Lord is transmitted to humanity. Literally Guru Nanak's body was a platform from which God Himself spoke and delivered His message- Gurbani (Divine Word). God manifested Himself through Guru Nanak:
'Gur meh aap samoai sabad vartaya.' 'In the true Guru (Nanak) He installed His Own Spirit Through him, God speaks’ Ang 127 SGGSJ
God is in the Guru and Guru is in God. Though God is everywhere and in everybody but His traits are illuminated through the Guru. The Jot (Divine Light) that enshrined Guru Nanak's body and the Primal Jot of God are, therefore, one and the same:
'Gur Nanak Nanak har soai.' 'O Nanak, Jot of Nanak and God are one.' – Ang 865 SGGSJ
When Guru Nanak conferred Guruship on Bhai Lehna (later called Guru Angad), the JOT was passed on and Guru Angad too became the embodiment of Divine Light. In the same way all the nine Gurus were the embodiments of Gur Nanak Jot (like a candle lighting another candle). The tenth Master, Guru Gobind Singh then conferred the Guruship on Sri Guru Granth Sahib Ji (Holy Scripture), which too became the embodiment of Divine Light. Gur Nanak JOT is, therefore, enshrined and preserved in Sri Guru Granth Sahib Ji. Sri Guru Granth Sahib Ji is the Living Guru for ever. For the Sikhs, the Sri Guru Granth Sahib is the manifestation of the Guru's Spirit.
When the Sikh Gurus were alive Sri Guru Granth Sahib Ji was then called Adi Granth as it had not been given Gurgaddi then due to Guru Ji being on earth in a physical form. Now Sri Guru Granth Sahib Ji is the total Guru, so to call Sri Guru Granth Sahib Ji - Adi Granth now is very disrespectful!
Sikhism endeavors to uplift the human soul from the shackles of Maya (materialism). It aims at a virtuous life which leads to the ultimate realization of a state of Eternal Bliss. The objective of Guru Nanak's Guruship was to give instructions in the True Name, to save humanity from immersing in the ocean of distress and misery arising out of worldly life, and to blend the human souls with their Creator, thus, emancipating them from the cycle of transmigration breaking all barriers and bonds of sufferings. This is the essential character of Sikh faith.