The Terraformer is the object placed on on Mars to create breathable air and habitable environment.
The Terraformer is the object placed on Mars by the Ultor Corporation prior to the events of the first Red Faction Game. it was put there to help cleanse the atmosphere and make air breathable without the need of environmental suits. It was also designed to make the environment more nature friendly, though we see minimal results of this in Red Faction: Guerrilla, it had begun.
Prior to the events of Red Faction: Armageddon, a disgruntled ex EDF soldier, White Faction and now Cultist leader, Adam Hale, leads a group of fanaticals to the Terraformer, with the intent of destruction via means of a pyrrhic victory. Hale is successful after distracting Darius Mason by impersonating a Red Faction unit, and blows up the Terraformer setting back the Atmospheres clock by over a hundred years.
Despite having the tools to repair the Terraformer, The Red Faction government intentionally covered up that it could be repaired because they realized it gave them more control over the Martian population.
The end of the Red Faction: Armageddon's campaign see's Darius restoring full functionality to the Terraformer after battling his way back to it after defeating Hale, and the plague of creatures Hale had released in an attempt to wipe out Mars inhabitants, regaining the atmospheric breathable air they had once had, but at the cost of most of Mars in the process.