Okay kiddo, so imagine we are playing a game and you have a special shiny sticker. You don't want just anybody to touch it or take it, right? That's where the "keeper of the seal" comes in.
In real life, some important papers and documents have a special seal on them that helps show they are official and important. The person who is in charge of keeping that seal safe and making sure only the right people can use it is called the "keeper of the seal."
It's kind of like being the protector of something important. They have an important job to make sure the document is kept safe and only used for the right reasons. Just like you want to make sure that special sticker stays with you and is used in the right way.