Promethean Giants are massive beings who existed before the rise of the New Gods. In their hubris, they sought to breach the Source wall and become one with the Source. For their transgressions, they were imprisoned by the Source in the Promethean galaxy, chained to massive celestial bodies, which served as their own machines for breaching the Source.

Two Promethean Giants were released from their imprisonment. 
 
One, Prodigian, was released by Mars, Roman God of war and Counterpart to Ares. He was drafted by Metron to release Darkseid from the Source wall by taking his place in it. He was, however, tricked by Mars into instead taking the place of Ares, who was also imprisoned there. To this day he still lies in the Source wall.
 
The second one, Gedirath, was outfitted with a mind control device by Darkseid and turned into a weapon. Darkseid used him to attack Orion who at the times possessed the anti life equation, but Orion was able to overcome him. What became of him is unknown.
 
Promethean Giants are absolutely massive in size, with Gedirath being able to hold the Earth in the palm of his hand without trouble. Given their massive size, they perceive time at a far slower rate than us. To Prodigian, his eons long imprisonment was nothing but a short incarceration, and the little chat Prodigian and Metron had took centuries in normal time.
Also as a result of their size, they are famed for their strength and power, which the gods such as Metron, Mars, and Darkseid have admitted. Powerful as they are, they are still nothing compared to the eternal Source.