Okay kiddo, "German Mediatisation" is a big word that describes what happened in Germany a very long time ago. Back in the year 1806, lots of little kingdoms and other places called "principalities" in Germany joined together to form a big group called the "German Confederation."
Now, before this happened, some of those little kingdoms were in charge of their own things, like owning land, making rules, and collecting taxes from the people who lived there. But when the German Confederation was formed, they wanted everything to be more organized and efficient, so they decided to take away some of those powers from the little kingdoms.
This is where "mediatisation" comes in. It means that the German Confederaion basically took away the power of the little kingdoms to rule themselves and made them a part of the bigger group. They also took away some of the lands and wealth that the little kingdoms had, and gave it to bigger and more powerful places like Prussia and Austria.
So, in simpler terms, German Mediatisation was when a bunch of little kingdoms in Germany joined together to form a bigger group, and the bigger group took away some of the little kingdoms' powers and gave them to more powerful places. It was a big change that helped make Germany more organized and united, but it also meant some places lost their independence and control.