Literally, a single flag. Figuratively used in expressions like '一旗揚げる' to mean attempting to make a success or start a venture.
彼は都会へ出て一旗揚げようと考えている。
He plans to go to the city and try to make his fortune.
祭りでは町中に一旗が翻っていた。
Flags were waving all over town during the festival.