Hickman's Avengers and New Avengers sets up the premise, but I dropped it all a long time ago so I'm not up to date and I got in fine.Is there much to know before going into the event?
Basically the initial premise is the Marvel multiverse is all out of whack, leading universes to collide, which usually results in the death of both universes. The Illuminati tried to fix the problem, but it basically became a choice of having to destroy one Earth to save the other. None of it really worked in the end, though. Leaving the Marvel and Ultimate Universe left, and they collide at the beginning of Secret Wars #1.
#2, I guess, will see the formation of the Battleworld, where we'll see the selection of the elements that will make it into the new universe.