The Grand Principality of Rus' was kind of like a big kingdom that existed a long, long time ago in a place called Russia. Back in 1658, people who lived in different parts of Russia were united under one powerful prince named Alexis Mikhailovich.
Think of it like this: Imagine your family living in different houses, but then your older brother becomes the boss of all the houses. Everyone listens to him and he makes the rules for the whole family. That's kind of like what happened with the Grand Principality of Rus'.
The people who lived in the Grand Principality of Rus' had their own language, religion, and way of life. They traded goods with other countries, fought battles to protect their land, and had their own laws and customs.
But this big kingdom didn't last forever. In fact, over time, Russia grew even bigger and more powerful, and eventually became what it is today. But the Grand Principality of Rus' was an important part of Russia's history and helped shape the country into what it is today.