Ok - if you go in for the parallel universe thing, it has to do with differences.
At every stage in everything, there's at least a couple of ways it can go, all the way from the quantum bit of a photon becoming either radiation or a particle, depending on who's looking, all the way up to your choice of what to put on your feet or have for dinner, depending on who's looking.
Each of these, from the small and insignificant to the huge and life changing, creates a different, "universe."
Universes with tiny differences (coin-flip universe, pirate universe, and the one where everyone looks like Leela and Amy and they never invented cloths) are, "close," to ours. Universes with BIG changes, like the ones (note plural!) where life never managed to get a foot hold, or the ones where the the matter in the universe was too energetic to coalesce are more, "distant."
Think of it as looking like a tree: A VERY BIG, VERY BUSHY tree. Something small has a possibility of turning out diffferently, and you get a little twiggy shoot. Something BIG can go two ways, and the tree bifurcates and each half takes off in different directions. In just a very few levels of choices and quantum differences, you have a pretty bushy tree!
In writing this little essay, for instance, every word I chose over one of its synonyms, every different sentence structure I chose, even choosing to write it, has created a different parallel universe.
You have to admit, though, the, "Naked Amy," universe would be a LOT more interesting than the, "Al chose, 'split,' instead of, 'bifurcate,'" universe.
Go to the library, find and read, "The Garden of Forking Paths,"
(oooh - BIG spoiler, there...) by the genius, Jorge Luis Borges
...Or don't. Create your OWN universe!