Original: The pejorative term 51st stater usually refers to non-U.S. residents who emulate mannerisms and culture of an American, or a non-American politician who is a supporter of the United States, especially its foreign policy, to a degree viewed as excessive.
Simplified: The term 51st state usually means non-Americans who like American culture and act like Americans, or a non-American politician who is a supporter of the United States, especially its foreign politics.