An "Old Karen" is a pejorative slang term for female over 50 who is an obnoxious, angry, entitled, homophobic, and often racist white woman who uses her privilege to get her way or police other people's behaviours. If this type of person has daughters they usually become "New Karens".
Old Karen didn't want a family from Africa moving into her gated community solely for racist reasons, so she devised a plan to have their application rejected.