Shield world
From Halopedia, the Halo wiki
The Shield world is a Fourunner construct built to shield them from the blast of the halo rings going off. They also seem to be related to Cortana's lines from the Halo 3 trailer. It would "shield" the Forerunners (or possibly Reclaimers) via a Mirco Dyson Sphere inside a Slipspace rift. This makes its a proverbial Forerunner 'bomb shelter' to the activation of the Halo instillations. Speculation: Although there is no mention of it, it would be logical to have more than one of these "Shield World", again only speculation.
