Okay kiddo, so you know how sometimes when you're playing with your toys, you accidentally lose one and you can't find it anywhere? Well, something kind of similar can happen with TV shows and movies.
Sometimes a TV show or movie gets made and shown on television, but then nobody takes good care of it and the tapes or film it was recorded on get lost or damaged. When that happens, people say that the TV show or movie is "missing," because nobody knows where it is anymore.
Now, sometimes people make copies of these TV shows or movies so that they can be shown later, even if the original is missing. But if ALL the copies of the TV show or movie are lost or damaged, then people say that the show or movie is "believed wiped." That means people believe that all the copies are gone and they can't be found anymore.
It's like if all your toys got lost or destroyed and you believed you couldn't find them anymore, even if they were really just hidden somewhere.
Sometimes, though, TV shows and movies that were once "missing believed wiped" are later found. People might find a copy of it in their attic or somewhere else, and then they are able to show it again. When this happens, it's pretty exciting for people who thought they would never get to see that TV show or movie again!