Multiverse

The Multiverse is the aggregate of all dimensions and parallel realities in existence. Although dimensions are just portions of reality within a universe, there are also parallel universes that coexist with one another. The Multiverse is believed to be explained by the eternal inflation system, however, the exact nature of its existence is largely unknown to its inhabitants.
Once the Multiversal War came to an end, a scientist named Nathaniel Richards chose a collection of universes that followed the same baseline and isolated it from the rest of the Multiverse. This collection was dubbed the Sacred Timeline and was assigned to the Time Variance Authority for protection. The Time Variance Authority's secret work was to prune and delete the timelines that branched off either by time travel or unscripted decisions by individuals who lived in the Sacred Timeline that could cause the arise of an evil version of He Who Remains which could lead to a new Multiversal War. After the death of He Who Remains at the hand of Sylvie Laufeydottir, numerous branches were created across different points in time, which paradoxically gave birth to alternate universes that had always been part of what individuals like the Ancient One and The Watcher knew as the Multiverse.
The Multiverse is always in fluctuation, and the several outcomes of certain events, which could result in the creation of alternate timelines, can be perceived and studied. Machines like Victor Stein's time machine and the Chronicoms' time stream were able to sense the nearest possible outcome within the timestream of the Multiverse. Realms where time is irrelevant, like the Quantum Realm, are a mechanism to travel either through time or access alternate timelines.
On Earth, to those who knew of the existence of the Multiverse this meant the existence of evil beings as Dormammu that could threaten the peace of their worlds was more than certain. Hence, organizations like the Masters of the Mystic Arts were created. Travel between dimensions of the Multiverse is also possible. Some places located in other dimensions, such as K'un-Lun, could be accessed from Earth only in certain periods of time, while some other dimensions, like the Mirror Dimension, can be visited through the use of Sling Rings. The interactions between dimensions and the Earthly plane in humanity's story and the danger it meant proved how the existence of the Masters of the Mystic Arts was more than necessary.