From https://en.m.wikipedia.org/wiki/Flag_of_Japan
Because I'm too lazy to type it out:
The national flag of Japan is a rectangular white banner bearing a crimson-red circle at its center. This flag is officially called the Nisshōki (日章旗, 'flag of the sun'), but is more commonly known in Japan as the Hinomaru (日の丸, 'Ball of the sun'). It embodies the country's sobriquet: the Land of the Rising Sun.
The sun plays an important role in Japanese mythology and religion, as the Emperor is said to be the direct descendant of the Shinto sun goddess Amaterasu, and the legitimacy of the ruling house rested on this divine appointment. The name of the country as well as the design of the flag reflect this central importance of the sun.
