Anglo is a term sometimes used in the United States to describe white people who speak English. It is sometimes used by social scientists, as well as casually.
It is a fairly useful descriptive term, and probably much less cumbersome than the alternative- "English speakers of European ancestry"- but it is also annoyingly inaccurate. "Anglo" derives from "Anglo-Saxon", and even in England, "Anglo-Saxon" would be an inaccurate ethnic description. In the United States, where a majority of the white population derives from European nations outside of England, it is even more inaccurate. Using the term "Anglo" also raises the issue of erasing or trivializing the differences between different European groups. It does not make sense to group people of Russian, Italian, Greek or Armenian heritage together as "Anglos", in opposition to Hispanic-Americans. And that does not even raise the issue of how the Irish might feel about having such a label applied to them!
However, this term is still used regularly, and it seems a silly thing to debate it, when a simple eye roll and teeth gritting will suffice.