In Halo lore, seven Halo rings make up the main array built by the Forerunners.
The installations are enormous ring-shaped megastructures designed as weapons against the Flood. When activated, they destroy sentient life with enough neural complexity to serve as food or hosts for the parasite. This starves the Flood and prevents it from spreading, but it also makes the rings an extreme last-resort weapon.
The original Halo array contained twelve rings, each 30,000 kilometres in diameter. The Forerunners later replaced it with a smaller array of seven rings, each 10,000 kilometres across. The games mainly focus on installations from this later array.
The number is often confused with the diameter of the rings or with the twelve-ring original design. Installation 04, Installation 05, and Installation 07 are individual members of the seven-ring array, not separate arrays.