Okay kiddo, let me explain what the Archduchy of Austria is!
You know how you have a family, right? Well, countries also have a family called a dynasty. The dynasty of Austria was called the House of Habsburg. They were a really powerful family in Europe a long time ago.
Now, within the House of Habsburg, there was a part of their kingdom called the Archduchy of Austria. It was basically like a big castle with lots and lots of land around it. The Archduke was like the boss of that castle and everyone who lived there.
So when people talked about the Archduchy of Austria, they were talking about that particular part of the kingdom that was ruled by an Archduke. It was an important place in Europe, and lots of people wanted to control it because it was rich and had lots of resources.
But over time, things changed, and Austria became a different kind of country with a different way of ruling. But we can still remember the Archduchy of Austria and how it was ruled by an Archduke.