To see your national flag in your dream signifies peace and/or prosperity. It may also bring about feelings of patriotism and duty to country. Or the dream is highlighting some political issue. If you see a flag of another nation, then it represents worldly issues.
To wave a flag in your dream symbolises a warning or distress signal of sorts. An issue may be weighing on your mind. In particular, a white flag indicates surrender. You are giving up. Seeing a red flag suggests your mercilessness and danger. The dream may be telling you to stop something that you are doing. To see a checkered flag in your dream represents success and completion.
To see a flag at half mast in your dream indicates that you are mourning. You are unable to let go of the past.