Billions of years ago, long before Earth was around and when Sol was a new star in the universe, the people of Oa (who would later call themselves Guardians of the Universe) were just beginning to scratch the surface of their newfound power. This power was utilized by Killala, the then-love of Dream of the Endless, and her people. This power was personified in Sto-Oa, the sun of Oa, and the being who stole Killala from Dream.
The Oans experimented with this power. The Glow manifested itself as a green fire when utilized by the Oans, specifically Killala. It would take eons for the use of the Glow to become refined in order for it to be wielded by the Green Lantern Corps.