Okay kiddo, so a long time ago, there were Kings and Queens who ruled over the land of France. They needed help to govern such a big place, so they gave some powerful people a special title called "Duke".
A Duke was kind of like a boss of a big chunk of land, called a "Duchy". The Duke had to make sure everyone who lived on their land was safe and happy, and that they followed the King's laws.
Sometimes the Dukes would fight with each other, or with the King, but mostly they worked together to make France a stronger and better place. Over time, the King became more powerful and the Dukes became less important, but they still had cool fancy titles that people remember today.