In Assassin's Creed lore, the ancient precursor species called the First Civilization is known as the Isu. The Isu existed on Earth before modern humans and created many artifacts that drive the series' stories.
Humanity was created or modified by the Isu for labor and other purposes. The two species eventually came into conflict, and a catastrophic solar event devastated the Isu civilization and most of humanity.
Artifacts such as the Apples of Eden and the Staff of Hermes are associated with Isu technology. Their influence survives through ruins, genetic memories, recorded consciousnesses, and visions encountered across different games.
The term “First Civilization” is an in-universe description, while “Isu” is the species' more specific name. They should not be confused with the Assassins, the Templars, the Hidden Ones, or the Order of the Ancients, all of which are later human organizations.