Okay, so a long time ago, there was a place called the Kingdom of Galicia-Volhynia. This was a big area of land that was ruled by a king.
Now, the people who lived there were called Ruthenians, but they also had other names like Ukrainians and Belarusians. They spoke a language called Old East Slavic, which is kind of like Russian and Ukrainian combined.
The Kingdom of Galicia-Volhynia was very important because it was one of the first places where the East Slavic people became organized into a kingdom. The king had a lot of power and controlled many things, like taxes, laws, and even the army.
The kingdom was also in the middle of important trade routes, which means that people would use it to transport things from one place to another. This made the kingdom very wealthy and important in the region.
However, over time, the Kingdom of Galicia-Volhynia started to lose power. Other countries nearby, like Poland and Lithuania, became stronger and started to take over parts of the kingdom. Eventually, in the 14th century, the kingdom was dissolved and its lands were divided up among other countries.
So, in summary, the Kingdom of Galicia-Volhynia was a big area of land ruled by a king, where the Ruthenian people lived and spoke Old East Slavic. It was important for its power and wealth, but it eventually lost power and was divided up among other nearby countries.