Nijūichidaishū is a big word that means "Twenty-One Successive Generations". It's a way of talking about a very special group of people, who are all related to each other and have all done really important things in Japan a long, long time ago.
Imagine if you were part of a family where every single person who came before you did amazing things - like being really brave samurai warriors, or famous artists who made beautiful paintings, or writing important books that everyone in the country read. That's what the Nijūichidaishū is like - it's a big family of all these very special people who all did really cool stuff.
The first person in the Nijūichidaishū was a man named Fujiwara no Kamatari, who lived over 1,300 years ago. He was a very important advisor to the king of Japan at the time, and he helped make some really big and important changes to the way the country was ruled. Over the years, other really important people were added to the Nijūichidaishū, like Minamoto no Yoritomo, who was a great general who helped to start a new government in Japan, and Taira no Kiyomori, who was a very powerful and rich leader.
The Nijūichidaishū is something that people in Japan still talk about today, because it represents all these amazing people who helped shape the country's history. It's like a big reminder that, no matter where you come from, if you work hard and do great things, you can be remembered and celebrated for hundreds of years to come.