Okay kiddo, so a long time ago there was this place called New Castile. It was very far away from where we live now. New Castile was a big area that had a lot of land and many people living there.
Now, in order to make sure things were running smoothly and everyone was following the rules, someone had to be in charge. That's where the governor comes in. The governor is like the boss of the area, making sure everything is going okay and that everyone is safe and happy.
So, the Governorate of New Castile was the name of the place where this governor worked. It was his job to make sure that everyone living there was okay and that everything was going smoothly.
Think of it like in our own school, we have a principal who is in charge of making sure we follow the rules and everything runs smoothly. The Governorate of New Castile was like a principal, but for a whole big area with lots of people!