Shield world

A Shield World is a Forerunner installation consisting of an entirely habitable Dyson Sphere and an artificial planet surrounding it. The Dyson Sphere resides either within a Slipspace transit (as in Onyx), or on the inner surface of an artificial planet (as with The Unnamed Shield World). These installations were built by the Forerunners in their vast empire, designed to be "bomb shelters" from the Halo Array's activation, but apparently were never used.

Dr. Catherine Halsey theorized that Shield Worlds were intended by the Forerunners as a form of a modern "bomb shelter," a place in which to ride out the devastating Halo pulse. As demonstrated in 2552, access to Shield Worlds was only possible upon the imminent activation of the Halo Array. Curiously, when Halsey and company fled into Onyx's Shield World during the last stages of the Battle of Onyx, it appeared to be deserted, implying that the Forerunners did not utilize it when the Halos were last activated. Also when a shield world shuts its shut for ever according to the novel Halo Ghosts of Onyx.

In Halo 3, these circumstances were revealed to be the result of the betrayal of the Forerunner AI Mendicant Bias, whose defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations.

Unlike the Halos, it is unknown how many Shield installations exist. It is possible that there are only seven of them (keeping with the seven reference). When the Forerunners built the Halos, it's possible they built an accompanying shield world with each. The Halos would destroy all sentient life within a 25000 light year radius of itself while the Shield World would protect any sentient life within that region.

Onyx
The internal Dyson Sphere resides in a slipspace bubble of compressed dimensionality, having a diameter of only a few meters in normal space, but vastly larger inside the slipspace rift. The Dyson Sphere contains environments suitable for Earth's sentient life. The only way in is a small portal, which is located inside the Core Room Antechamber in the core of the artificial planet.

Unnamed Shield World
This Shield World differed greatly from the base concept of the Shield Worlds, being only a hollow artificial planet with a terraformed interior, instead of having a massive Dyson Sphere inside a slipspace rift. Accessing the interior was also easier than on Onyx, as the Shield World had multiple access tunnels around the surface. Its function was also apparently different, housing a fleet of Forerunner Ships instead of being a bomb shelter.

Shield Worlds and the Covenant
The Covenant owe their rapid technological advancement to the discovery of a small number of other Shield Worlds, discovered earlier in their history and stripped of usable technologies. The eventual fate of these worlds, and whether they were abandoned or inhabited by the Covenant, is unknown.

One of these worlds contained a fleet of Forerunner warships, which the Covenant planned to use in their war against the humanity. Before they could accomplish this, the crew of the UNSC Spirit of Fire destroyed the Shield World by overloading the ship's Shaw-Fujikawa Translight Engine in the sun in the center of the shield world. This caused it to go supernova, thus turning the entire structure into dust, preventing the Covenant from using the Forerunner fleet.

Trivia

 * Worlds like Onyx were referred to as "The Shield" of the Forerunner, while the Halos were "the Sword." This echoes Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword." So far, Bungie has said nothing about a correlation between Cortana's use of the word "Shield" and the Forerunner's "Shield World."
 * A "shield installation" is mentioned in the cutscene on the level "The Ark" in Halo 3. 343 Guilty Spark states that he had hypothesized that the Ark might be located within such an installation, but was proven wrong by the discovery of its location on the exterior of the galactic rim. This also brings up the possibility of many Shield Worlds existing, further assisted by the the Flood-controlled Shield World that appears in Halo Wars
 * The Shield World's glyph can be seen in the cover of Halo: Ghosts of Onyx, hinting and supporting that the novel was to focus mainly on the subject. There is quite an uncanny resemblance between this glyph and a glyph seen on the world discovered at the end of Halo 3 when completed on legendary, which may confirm that Master Chief and Cortana are heading towards the/a Shield World, or rather, the Forerunner construct that might lead to the Shield World (like the object on Earth that was originally considered to be the Ark).
 * In Terminal 5 of Halo 3, there is a sentence that reads the following "If we start immediately--commence total biosphere elimination of life sustaining worlds (as indicated in the accompanying charts) and relocate evacuated populations to facilities such as those described in the [Onyx project] all this could be achieved in [57,1590 (+/-2,184) hours]." This is most likely a reference to either the Forerunners Shield World(s) plan or Onyx itself.
 * In Halo: Ghosts of Onyx, the planet Onyx is actually a Forerunner-manufactured planet which is a composite of Sentinels. Within the planet’s network of underground tunnels, a controlled slipspace rupture allows individuals to move into a micro-Dyson sphere hidden in subspace at the core of the planet. The sphere’s interior surface, similar to what can be seen in Halo Wars, is considerably larger — so large in fact, that the individuals trapped within it didn’t even realize they were in such a super-structure. However, in Halo Wars, the Shield World is simply a planet shell, its outer surface covered with large bodies of water and the Flood parasite, presumably having spread by way of the Covenant intrusion.