Okay kiddo, so imagine you're playing a game with a bunch of your friends, and you all have different positions and titles. Like maybe Susie is the queen and Billy is the knight. Well, a long time ago in a place called Slavonia, there was a man who was the boss of all the land and the people who lived there. He was called the duke of Slavonia.
The duke was kind of like the leader of Slavonia. He was in charge of making sure everything ran smoothly, like making sure people had enough food to eat, that the roads were safe to travel on, and that everyone followed the rules. He also had some soldiers who would protect the people and the Duke's castle from any enemies who might try to attack.
Being the duke was a really important job, and it came with a lot of privileges, like living in a big fancy castle and having lots of money and power. But it was also a hard job, because if anything bad happened to the people of Slavonia, the duke was responsible for fixing it.
So that's what a duke of Slavonia was - just a really important person who was in charge of taking care of a whole kingdom a long time ago. Cool, huh?