Respect the flag is a teem coined by the twitter user @seantheterrible. It references him tweeting he heard "Fuck you, Ni**er" instead of "Respect the flag" when the phrase was mentioned in conversation.
"Respect the flag" was adopted by forums, such as r/4chan, twitter, the original 4chan and various subgroups as a way to insult people of colour.
NormalUser1: RESPECT THE FLAG
NormalUser2: the flag?
NormalUser1: Yes, respect it