Ultras are basically people who demand immediate utopia and want to disregard all of history. They disregard current material conditions and believe the working class can simply will communism into existence whenever they feel like it.

When ultra is used as a pejorative though it means something like purist. Like someone who believes in a pure, untainted version of communism that exists outside of any historical movements or current political formations. “True communism has never been tried” is their catchphrase.