So, imagine you're living in a big castle a long, long time ago. People spoke different languages back then, like English or French or German. But when people who spoke different languages wanted to talk to each other, they used a special language called Latin.
Now, as time went on, Latin started to change. People in different parts of the world started to speak it a little differently, and they even added new words to it. This version of Latin that had changed is what we call "medieval Latin."
Medieval Latin was still used lots of important things, like for writing books or legal documents. But it was different from the Latin that people used in ancient Rome. Just like the English that we speak today is different from the English that people spoke 500 years ago.
Medieval Latin stuck around for a long time because it was still very useful. But eventually, people stopped using it as much because other languages (like English) started to become more popular.