In Halo 3, the Forerunner installation revealed at the end of the campaign is the Ark.
The Ark is a massive Forerunner facility located outside the Milky Way galaxy. In the game's story, the portal beneath New Mombasa provides access to this distant installation, allowing the Master Chief, the Arbiter, and their allies to pursue the Prophet of Truth.
The Ark is important because it can activate the Halo rings remotely. Truth attempts to use it to fire the array, while the protagonists try to stop him. During the final stages of the campaign, the Flood also reaches the installation aboard the crashed High Charity.
The Ark is not the same object as Installation 04, the ring featured prominently in Halo: Combat Evolved. The replacement ring built at the Ark is destroyed when it is activated prematurely, while the damaged Ark survives in the immediate aftermath. Later Halo stories expand the installation's role and identify it as the location where the Halo array was constructed and controlled.