You may flag down a police officer if you have a problem and you need his help or her help.
如果您遇到问题并且需要他或她的帮助，您可以举报警察。
英语百科
Flag officer
A flag officer is a commissioned officer in a nation's armed forces senior enough to be entitled to fly a flag to mark the position from which the officer exercises command.
The term is used differently in different countries: