I will try to paraphrase something from an Asimov short story.
Think of the universe as infinite. If this is true than there must be infinite worlds with life EXACTLY like ours and every possible situation happening on them. If you are walking up a hill with your friend and go around a corner....then on infinite other worlds there MUST be two people who look just like you, know everything you know, have the same names, and have lived the same life as you, walking up a hill that is EXACTLY like the one you are, with infinite outcomes around that corner. Anything you can imagine must be happening on infinite worlds in the same scenario you are in...for example; a 3 eyed giant purple mud sloth will be waiting around a corner to eat the two people on an infinite number of those worlds...and so on.
Kind of hard to fathom, but it is probably the best description I have ever heard. Of course Asimov put it alot better than that, being that he was a literary genius.