The Multiverse is the collection of alternate universes that share a universal hierarchy; it is a subsection of the larger Omniverse, the collection of all alternate universes. A large variety of these universes were originated from another due to a major decision on the part of a character. Some can seem to be taking place in the past or future due to differences in how time passes in each universe. Often, new universes are born due to time traveling, another name for these new universes is an "alternate timeline".
The established "mainstream" universe in which all the main books and stories take place in is Earth-616, but for the purposes of allowing users to have meaningful roles in the events in the Marvel Universe, this RPG takes place in an alternate earth, dubbed Earth-2112. Earth-2112 is functionally identical to Earth-616 at its creative peak in the 70's and 80's, before the universe was marred by weekly world-shattering events and meaningless character deaths, but with modern aesthetics and characterization, and the room for retellings of major crossover events later down the line.
By establishing our own universe, we are able to distill certain storyarcs and plots and remove some of the largely disliked events from our own canon, while also allowing for the adaptation of other Marvel sources- like the super popular Marvel Cinematic Universe or the multitude of Marvel-themed video games.