Okay, so there is this really cool TV show called Doctor Who where the main character is a time-traveling alien called the Doctor. The show has been on for a really long time, since the 1960s, and some of the episodes from the earlier seasons are missing.
Now, missing episodes means that the recordings of those episodes were lost or destroyed somehow. This happened because back in the old days, TV shows were recorded on these big film reels that took up a lot of storage space. So, when the shows were done airing on TV, the film reels would often get thrown away or reused for something else.
Sadly, this happened to a lot of the early Doctor Who episodes. But don't worry, people are still looking for them! Some episodes have been found in other countries where they were shown on TV, and some episodes have been reconstructed using pictures and audio recordings.
So, while it's sad that some of the Doctor Who episodes are missing, it's also kind of exciting because it means we might find them someday! And in the meantime, we can still enjoy all the other adventures that the Doctor goes on through time and space.