I am probably not the best one to try and do that, Bruce maybe. But basically it's...alternate dimensions and/or universes. I think someone called this a pocket universe once? It's some sort of magic I won't even try to understand that pulls people here from all different worlds and places and times, sometimes more than once, and dumps them here. We don't know how we get here or how to get home but the world we came from still thinks we're there too.