The end of Avengers: Endgame certainly wrapped up the whole drama with Thanos and his Infinity Gauntlet. However, in doing that it set the rest of the Marvel Cinematic Universe on a brand new, uncharted course as we move forward into new movies.
Not only are we now five years in the future (it’s supposedly now 2023), but the idea of a Marvel “multiverse” is just being dropped into the MCU now. You know, that moment when which Marvel let Jake Gyllenhaal's Mysterio drop the multiverse line in the Spider-Man: Far From Home trailer. What’s the multiverse, you might ask? Oh, it’s just the idea that there are other parallel dimensions, universes, and alternate timelines. They were doing perfectly fine on their own, but thanks to the snap, and all the branched timelines that the Russo Brothers keep confirming, the possibility of the multiverse affecting the MCU is, well, possible.
The easiest way to describe the multiverse is that the same characters, heroes, villains, places, people, and things can exist simultaneously in different dimensions never crossing paths with the others, unless something happens — like say, a snap of the Infinity Gauntlet and the Avengers going back in time to try and collect the Infinity Stones and Loki taking the tesseract and disappearing.
In doing this, a multiverse was theoretically created, which is why the new guy in Spider-Man: Far From Home, Mysterio, can tell our Spider-crew, quite casually, that he's from a different dimension. He says the bad guys in Far From Home, the Elementals, are from his world and he needs Peter Parker’s help to try and stop them, and we can all accept that because time got weird in Endgame.
The multiverse being mentioned in Far From Home opens up a lot of potential for the MCU. Director John Watts told Fandango that when making Spider-Man: Far From Home, “We had to look at it in terms of the scope of what happened at the end of Endgame. Seeing all the crazy things that they did and all the questions that raises. So we're definitely trying to answer one of the big ones — alternate timelines. So many possibilities opened up at the end of Endgame, and Peter Parker is one of the few people on the ground dealing with them.” Then when asked to describe how the multiverse works here, Watts declined to do so. (Those who've already seen Spider-Man: Far From Home now know why.)
Whatever role the multiverse plays in Far From Home, it's a term that fans should wrap their heads around after Endgame changed the game. There’s one comic storyline, Secret Wars, that has characters from all different dimensions coming together (and the Russo Brothers have already expressed interest in doing it), but that story would only be possible if the multiverse plays a big role in the MCU going forward (along with some characters that are available to the MCU following Fox’s merger with Disney). Though Secret Wars hasn’t been hinted at whatsoever in any movie, we are heading into Phase 4 after Far From Home, and anything’s possible. Not much is known about what happens next, but the multiverse might be the key to everything.
If this all still sounds widely confusing, the easiest way to grasp this concept is just to watch the Oscar-winning Spider-Man: Into the Spider-Verse. That movie — though animated, from Sony, and completely separate from all other Spider-Man stories — deals with the multiverse, too, which is why there are so many different versions of Spider-Man in the movie.
And that’s basically the tl;dr gist of the multiverse: It means there are other versions out there that we don’t necessarily know about or need to concern ourselves with (you know, unless someone out there builds another Infinity Gauntlet).