A service flag is a banner displayed by family members of those serving in the United States Armed Forces. The flag or banner is officially defined as a white field with a red border and a blue star for each family member serving in the Armed Forces during a period of war or hostilities. A gold star represents a family member who died during military operations.

Based on the star symbols used on the service flag, the term "Blue Star" has come into use in the United States as a reference to having a family member in active military service, while the term "Gold Star" has come to refer to the loss of a family member in military service. For example, the mother of a person who died in service is referred to as a "Gold Star mother," and the wife of an active service member is referred to as a "Blue Star wife." Charitable support organizations have been established for Gold Star mothers, Gold Star wives, Blue Star mothers, and Blue Star wives. The last Sunday in September is observed as Gold Star Mother's Day, Gold Star family members are entitled to wear a Gold Star Lapel Button, and all 50 U.S. states and Guam offer some form of a specialty license plate for motor vehicles owned by Gold Star family members.

From Wikipedia