The United States Minor Outlying Islands are nine insular areas of the United States, almost all of them remote and unpopulated. They have no flag of their own. The flag flown over them is the national flag of the United States.
Nine islands taken for guano and for war
The group consists of Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Atoll, Navassa Island, Palmyra Atoll and Wake Island. The United States acquired them at different times and for different reasons, mostly military position and, earlier, guano.
The Guano Islands Act of 1856 let U.S. citizens claim islands with guano deposits, and several of the territories now in the group came into American hands that way. Guano was in heavy demand as agricultural fertilizer through the 19th century.
Two of the islands are better known for what happened on them during the Second World War. Wake Island was a military base and an early Japanese target. Midway Atoll gave its name to the Battle of Midway.
The flag they actually fly
None of the nine has an official flag of its own, so the U.S. national flag stands in: thirteen horizontal stripes alternating red and white, with fifty white stars on a blue canton. Red is usually read as valour, white as purity and innocence, and blue as vigilance, perseverance and justice. The fifty stars are the fifty states. The territories are not states, but flying the same flag ties them to the rest of the country.
Flag protocol on the islands follows the same rules as anywhere else in the United States, at the handful of government buildings and military installations that exist. Unofficial badges and emblems occasionally stand in for individual islands, but none of them has any formal status.
Insular areas and unincorporated territories
Two terms come up constantly in writing about these islands. An insular area is a U.S. territory that is not part of any state. An unincorporated territory is one where the U.S. Constitution does not apply in full and no organised local government has been established.
In practice the islands are managed by federal agencies, chiefly the Department of the Interior. They sit outside the usual structures of statehood, which shapes both how they are governed and which federal laws reach them.
References
[1] U.S. Department of the Interior (doi.gov)
[2] U.S. State Department - Insular Affairs (state.gov)