Okay little one, the Crown Jewels of the United Kingdom are like super special shiny things that belong to the queen and the royal family. They are really, really old and valuable things like crowns, sceptres (a fancy stick), and swords that are used in special ceremonies like coronations and weddings.
These special shiny things are kept in a big castle in London called the Tower of London. It's guarded by a group of people called Beefeaters, who make sure that no one steals the Crown Jewels or tries to mess with them.
When the queen needs the Crown Jewels for a special event, she gets to take them out of the Tower of London and use them for the day. Then, they are carefully put back in their special cases and taken back to the Tower of London where they stay safe and sound until the next time they are needed.
So, in short, the Crown Jewels are special, valuable, and very old things that belong to the queen and are used in important ceremonies. They are kept in the Tower of London and are guarded by Beefeaters to make sure they stay safe.