Sangokujin is a Japanese word that means "people of three kingdoms". It refers to people from China, Korea, and Japan, which were three kingdoms in ancient times.
So basically, back in the day, there were three big kingdoms that ruled over areas of Asia: China, Korea, and Japan. People who come from these places are called Sangokujin.
Think of it like a big club that has three different groups, but they all have a special bond because they come from the same area of the world. It's like if you and your friends all come from the same neighborhood, you might feel connected in a special way.
Even though these kingdoms don't exist anymore, the term Sangokujin is still important because it acknowledges the shared history and cultures of these three countries.