It is often either sung or performed during or immediately before the raising or lowering of a flag during a ceremony.
Most countries use their respective national anthems or some other patriotic song for this purpose.
[1] However, some countries, particularly in South America, use a distinct flag anthem for such purposes.
Flag anthems can be officially codified in law, or unofficially recognized as such through mere custom and convention.
In some countries, the flag anthem may be just another song, and in others, it may be an official symbol of the state akin to a second national anthem, such as in the Republic of China (Taiwan).