Okay kiddo, let me explain what the Duchy of Rome is.
You know how when we play house or pretend to be kings and queens, we have a pretend kingdom with a leader who is in charge of everything? Well, back in olden times, there was a real place called the Duchy of Rome which was ruled by important people called dukes.
The Duchy of Rome was a big piece of land that included the city of Rome, and it was ruled by dukes who were chosen by the Pope. The Pope was the leader of the Catholic Church, and he was very powerful and important.
The Duchy of Rome was a special place because it was the center of the Catholic religion. People from all over the world would come to Rome to see the Pope and visit important religious places like St. Peter's Basilica and the Vatican.
So basically, the Duchy of Rome was an important place ruled by dukes who were chosen by the Pope, and it was considered very special because it was the center of the Catholic religion. Does that help you understand, my sweetie?