We kinda did describe it.
However, here is a basic breakdown without fanfare:
Rampage is where you get to play around with UE, Money, skills, and you gain A TON of UE to put into skills and stats and you use a lot of money for stuff and you can use Rampage to help you learn the game and see what things do.
What certain skills do.
What pieces of tech do that you might not have been able to use before.
What weapons do.
What the vehicles do.
Meet players you might otherwise not meet (though Rampage is an alternate universe and anything you learn in Rampage does NOT cross over back into regular SD). :)
You log in like usual but it's an alternate SD universe where something like Resident Evil, 7 Days to Die, All zombies must dies video games have happened, or, if you like tabletop games, Zombies!!. And the expansion for Zombies!!.