The Archons, also known as The Seven, are seven beings of immense elemental power who have risen to the point they are worshipped as gods by the people of Teyvat. We so far have met two of them in the game, Venti and Zhongli.
Venti is the “Anemo Archon” making him the god who rules over the wind element, also known to the public as Barbatos. Zhongli is the “Geo Archon” who rules over the earth element, also known publicly as either Rex Lapis (a title more than a name) or Morax. Both of them came to power during the “Archon War” that took place hundreds of years before the events of the game, though it is notable that (according to Zhongli) none of the other members of the original Seven are still alive, the remaining archons being considerably younger than the two we have met. The title is one that can be earned or given, as it and the power that comes with it is tied to an artifact carried by the Archons inside their bodies, called a “Gnosis”. The Gnosis kind of looks like a chess piece and allows the Archon to use their full power as an Archon, but it has been shown that they still retain a fair amount of power even without it. As they are also elemental beings themselves, they have powerful abilities related to their element separate form being Archons. The Gnosis is supposedly a gift from Celestia, the world of the creator, marking those who possess it as an Archon, basically making it a god-tier version of a Vision that imbues mortals with elemental power. According to some sources, the ability to grant mortals a Vision is also an ability the Archons have by using their Gnosis. For reasons currently unknown, the Cryo Archon of Schneznaya has ordered her servants to begin collecting the other Archons’ Gnosis. Venti’s was stolen by Signora of the Fathi while Zhongli willingly gave his to Signora as part of a “contract to end all contracts” as he wished to retire. Notably, Zhongli claims another Geo Archon will likely replace him in 100 years or so, though he makes no note of what may happen to his Gnosis.
While we do not presently know how Zhongli came to have his title, we do have part of the story of how Venti got his role. He was once a normal wind sprite who lived in Mondstadt at a time a different wind god, Decarabian, ruled the place. Venti, alongside several humans and other spirits, waged war against Decarabian and won, claiming freedom for Mondstadt. Venti, under the name Barbatos, had claimed the title of wind god afterward and received his Gnosis, becoming the first Anemo Archon to rule over Mondstadt. Wishing to preserve the freedom he and his allies fought for, Venti abdicated the position, allowing mortals to rule themselves in his territory and only ever stepping in when some sort of calamity occurs.