the really super short is that the multiverse is collapsing one universe at a time (this is why them using the term "incursion" was so important in Dr Strange 2). The 616 tries to stop it but ultimately fails and everyone dies. But behind the scenes Doom was secretly working on a plan and basically cobbles together bits and pieces from all the realities into his own, where he installs himself as god and rules over everything.
It was the big conclusion to one continuous story running for 8 years thru Fantastic Four, Avengers, plus a shit ton of other comics (all written by Jonathan Hickman) and eventually culminated in Secret Wars. To give you an idea of how big of a deal it was treated, Marvel literally cancelled every single series they had when it kicked off. They made a bunch of spinoff titles related to the main series, but while Secret Wars was happening the entire comic universe was nothing but that storyline, everything else was stopped.