Summary
The Forerunners constructed seven massive rings, called "Fortress Worlds". They are artificial worlds complete with gravity and different environments such as deserts, oceans, islands, arctic-like tundras, mountain ranges, and swamps. Called "Halos" by the Covenant, they were designed to study and contain the Flood lifeform, as well as function as weapons of mass destruction. They were created a few hundred years after the Flood was discovered by the Forerunners, because their builders saw that their current isolation and containment procedures were not adequate to stop the spread of the Flood.
The Halos were built with an extensive teleportation grid that covers the entire ring. It is unknown exactly how the grid digitalizes matter and transports it, but it is theorized that it may be simply a miniature, and more precise version of slipspace technology, in that it transports matter in the same way UNSC and Covenant spacecraft travel through the galaxy.