The Outerverse is the current prison and home of the Ogdru Jahad, after being banished by God far beyond the boundaries of all that He reigns sovereign in before being established as their prison to keep them at bay, as they could not be destroyed.
The nature of the Outerverse is largely unknown, but what one hypothesis is that it acts as the space which is outside of God and Asherah's circle of creation meaning that Creation itself is compacted into a various layers of cosmic rings with the Outerverse being its own layer that took form while being exempt from God's in the void. Another theory is that it took form around the eldritch deity known as Yog-Sothoth, who also acts as all of time and space in this verse as well.
Despite holding some of the most powerful beings at bay, lesser versions of their kind can leave the realm to varying extents; the deity Nyarlathotep appears to able to escape the Outerverse whenever he pleases, and Shub-Nibburath can be summoned via an occult ritual.
While created as a prison, the strongest of the Ogdru Jahad were strong enough to remold the prison as a "home" for their race including the newly created Outer Gods like Yibb-Tstll, Abhoth, and others which were all created in this realm. They are able to do this even while being imprisoned within their Golden Cocoons although it is likely that their presence alone was able to shape the Outerverse into what it is perceived as similarly to how Lucifer and his fallen cohorts shaping the Abyss into Hell with their own presence.
Physically the Outerverse is described as "black starless void", and the various Ogdru Jahad are trapped in golden, chrysalis-like cocoons. They are also many tears in reality that allows some of these Chaos Gods to escaped the prison and enter the multiverse. The Outerverse can be only be really opened with the Right Hand of Doom, the key that God created to keep the gates to the outer realm closed. The key is currently grafted onto the cambion Hellboy, who works for Bureau for Paranormal Research and Defense.