The Ryukyu Kingdom was a place a long long time ago (like older than your grandparents!) It was located in a group of islands between Japan and Taiwan called the Ryukyu Islands.
The kingdom was ruled by a king who lived in a big palace with lots of other people who helped him run the country. The king had lots of power and made decisions for the people who lived in the kingdom.
People who lived in the Ryukyu Kingdom were called Ryukyuans. They had their own language and culture, but they were influenced by lots of other cultures too like those of China, Korea, and Japan.
The kingdom was really important for trading (kind of like buying and selling things), especially because it was in a good spot between lots of other countries. People from all over the world would come to the Ryukyu Kingdom to trade things like silk, ceramics, and spices.
Sometimes the Ryukyu Kingdom would get in fights with other places, like China and Japan, and sometimes they would make friends and work together.
The kingdom lasted for a really long time (over 500 years!) but eventually, it was taken over by Japan and became part of their country.