In Halo: Combat Evolved, the mysterious Forerunner installation discovered by the UNSC is Installation 04.
Installation 04 is one of the ring-shaped superweapons built by the Forerunners. It is usually called Alpha Halo by fans and reference materials, because it is the first Halo installation encountered by players in the main game series.
The ring becomes the central setting after the UNSC ship Pillar of Autumn exits slipspace near it. The Covenant also seeks the installation, believing that activating the rings will begin the Great Journey. Master Chief and Cortana eventually discover that the structure is designed to eliminate sentient life across a vast region of space, starving the Flood of hosts.
Installation 04 is destroyed near the end of the game when Chief detonates the Pillar of Autumn's fusion reactors. Installation 05, seen in Halo 2, is a different ring.