The multiverse (or metaverse) is the set of multiple possible universes (including the historical universe we consistently experience, Earth Prime) that together comprise everything that exists: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The various universes within the multiverse are sometimes called parallel universes.
Think of Michael Moorcock’s version: “The Multiverse is a series of parallel universes in many of the science fiction and fantasy novels and short stories (written by Michael Moorcock) There are many other fictional settings also have the concept of a Multiverse. The Multiverse contains a legion of different versions of Earth in various times and histories. The version here in Glatheriel borrows from an early D&D supplement/game system, the Arduin Grimiore Trilogy by David Hargrave.