Pan-Germanism is a big idea that some people in Germany had a long time ago. They thought that all German-speaking people should live in one big country called Germany.
Let's pretend that you live in a big house with your family, and your cousin lives in a smaller house down the street. Your cousin speaks German just like you, and you like to play together.
Now, imagine that some grown-ups in your family say that it would be better if you and your cousin all lived in one big house. They think that it would be easier to share things like food and toys, and you would all be safer together.
That's kind of what Pan-Germanism is like. The grown-ups in Germany thought it would be better if all German-speaking people lived in one big country. They thought that everyone would be stronger and happier together.
But not everyone in Europe at that time wanted to be part of Germany. Some people in other countries spoke German too, but they also had different ideas and traditions. They didn't want to give up their own ways of doing things just to join Germany.
So, Pan-Germanism was a big idea that some Germans had a long time ago. They wanted to bring all German-speaking people together in one big country, but not everyone agreed that it was a good idea.