Lost media means stuff that we used to have or see, but we can't find them anymore. It might be an old TV show, a movie, a book, or even a song. Sometimes things get lost because they were never saved properly or because the things that held them got destroyed. Other times, people might just forget about them or lose them.
Sometimes, we only have little bits and pieces of lost media, like a short clip or a picture. When people try to find lost media, they have to use different methods to try and locate them. Sometimes they talk to people who might have seen or own them, other times they might look through old archives or libraries.
It's like trying to solve a big puzzle with missing pieces. The people who are looking for these lost things are like detectives who are trying to solve a big mystery. When they finally find a lost movie, book, song or TV show, it's like they have found treasure that everyone thought was lost forever.