Multiverse

The multiverse is the existence of many universes, dimensions, pockets, alternate realities that exist in the overall universe.

Background
Hell has conquered many worlds across the multiverse in its war to conquer all.

''Within the hub of the Doom Slayer's chamber. VEGA monitors quantum abberations with great interest. As the forces of Hell and Urdak collide with the mortal realm, the tenuous order of reality begins to fragment, signaling the dawn of a new age of chaos. ''

A dedicated portion of VEGA's simulation matrix observes, records and analyzes these dimensional abnormablities as VEGA attempts to comprehend their meaning. Processing a veritable  kaleidoscope of  hyperfluxual phenomena, VEGA observes what may very well be the result of the multiverse imploding on itself, where countless battles are fought between the Doom Slayer and demonkind.

To further explore this anomaly, VEGA has designed an onboard omni-contextual interface, an interdimenional man-machine gateway that allow the Slayer to engage in these skmirshes directly. As the Slayer enters the gateway, his surroundings take the form of an arena, in tandem with the arrival of adversaries ready for battle Seemingly extracted from the data of his experiences, the opponents the Slayer faces are familiar but now transformed by some unseen force.

With further analysis Vega observed ephemeral distorations within the fabric of reality; scrambled reflections of the physical world. As the slayer interacts with this anomoly, a peculiar ripple in the anomaly's composition emerges, seemingly reacting to the Slayer's presence reflexively. The anomoly reshapes itself, calibrating itself to the Slayer, responding to his memory, ancestral, and memetic data, before ultimately projecting itself onto the Slayer himself. 

Behind the scenes
This lore about 'multiverse' is in a section  of the Art of Doom Eternal pertaining to odds and ends like artwork for collectables, easter eggs, costumes, multiplayer (characters/arena maps), and various skins for the Battlemode "transport Gate" (the background seen when changing appearances of the Slayer, of his weapons, or of his enemies). Some future skins will apparenly be based on other ID characters and franchises.

This appears to be a lore based explanation and reference to a number of game mechanics and easter eggs. The first is it appears to be describing Battlemode/Multiplayer games, secondly it appears to be referencing the Doom Slayer having lived many lives(potentially explaining the Extra-life mechanics), it also explains Player Customization features in the games he's appeared in, it also explain 'easter eggs' and things he has laying about his office: collectables, costumes, gate changing feature, all of which he may have pulled from worlds across the 'multiverse'. It also hints that previous games he's appeared in are all versions of himself, or his own ancestors (B.J. Blazkowicz, for example) or versions of himself in alternate universes (ex. Flynn Taggart). Through quantum mechanics of multiverse, he is able to touch into other worlds he existed in, and remember their memories.

See also ID multiverse timelines and spacetime.