The concept is simple: Suppose the referee flips a coin to determine which team kicks off. It could come down "heads" or "tails" ,and the result of that coin toss might help determine which team wins.
So far, so good; but here's where it gets complicated. The "parallel world" people say the mere fact the coin was tossed at all creates at least two parallel worlds : one in which Team A wins;the other in which Team B wins. The two worlds exist side-by-side;but things work out very differently in them. (The Captain of the losing team doesn't get to marry the girl, or to father the child, who will grow up to be the doctor who saves the life of the Prime Minister,who was the last hope for peace,etc.,etc.)
In case you're wondering, I know all this because I read it in a science fiction story,at a time when I should have been studying Intermediate Algebra,so I could pursue a career in Chemistry,and invent the formula that would have saved millions from starvation,and..and...
I was a bit startled. "Where the heck did YOU come from ? "
Ms Rabbit shrugged. " I came through the Time Portal, of course !"
She tossed her head ."You claim to be the "expert". Didn't you know about Time Portals ? "
"Whoa ! Whoa !!", I almost shouted. " You're saying there ARE parallel universes ? That there are ....Time Portals that...let people go back and forth ?"
Ms Rabbit twitched her nose. " I thought EVERYBODY knew that ! For instance, where do you think Mr. Meangenes is right now ? "
I spun around. He had vanished - just like that !
Ms Rabbit looked at me pityingly. " He decided to visit the universe where you DID remember to pick up a six-pack of Yuengling for him. You know how he likes his beer !"
" People go back and forth all the time. That's what causes half the problems in the world. For example, Britney Spears is planning to convert to Islam, start wearing a burkha, and marry some Muslim gentleman...which might have been a good plan in her universe,but isn't getting rave reviews in this one. "
I closed my eyes in disbelief. " This whole ...parallel universe/time portal business is giving me a headache ! You've got me so discombobulated, I don't know what in heck to believe in any more !"
"That was good (Urp !) beer ! I'll have to try that Time Portal business more often."
He began to stroll off. "By the way," he said. "Did you know there may be universes in which there are NO Time Portals, and People DON'T talk to rabbits ? "