flag with a red circle on a white background
Describes the physical design: a white field with a red disc. Often used when referring to the flag as an object or symbol.
At the sports day, the children were waving small flags with a red circle on a white background.
Specifically the national flag of Japan, also known as 日章旗 (にっしょうき). Used in official or patriotic contexts.
See also: 日章旗
On national holidays, some households display the national flag of Japan in front of their houses.