The Titans, in Greek mythology, were the children of Uranus (heaven) and Gaea (earth), and of gigantic size. They engaged in a conflict with Zeus, the king of heaven, which lasted ten years. They were completely defeated, and hurled down into a dungeon below Tartarus.
Very often they are confounded with the Giants, as has apparently been done here by Pope. These were a later progeny of the same parents, and in revenge for what had been done to the Titans, conspired to dethrone Zeus. In order to scale heaven, they piled Mount Ossa upon Pelion, and would have succeeded in their attempt if Zeus had not called in the assistance of his son Hercules.