Daijō tennō is a Japanese title that a really long time ago in Japan, people used to call their kings or emperors. Think of the daijō tennō like a really important king who everyone listened to.
Back then, Japan didn't have just one ruler. There were actually many people who ruled different parts of the country. But if someone became the daijō tennō, it meant that they were the most important person in all of Japan.
Being the daijō tennō was a really big deal. They would have lots of people working for them, like servants and soldiers. They would have fancy clothes and live in a big palace. And people would always treat them with a lot of respect.
Today, Japan doesn't have a daijō tennō anymore. But people still remember how important they were in Japanese history.