Ask Lemmygrad

Ultra-leftism can take different forms. Ultra-leftists don't correctly analyze and understand social conditions, so they criticize socialist countries for not already having progressed further toward communism (and often actively oppose them for this reason), and they criticize socialists for speaking to the people in the context of their own social conditions instead of pretending the people are ahead of where they are. They'll say things like that you're not a real socialist if you're not physically fighting agents of the capitalist state or only speaking about core issues like class (instead of what the people care about). They're often fixated on particular revolutionary leaders or particular kinds of direct action instead of real-life organizing based a serious and informed analysis of current social conditions.

Left-Wing Communism: an Infantile Disorder by Lenin is great but may take some extra time and effort and additional resources to understand if you're new to studying topics like this or unfamiliar with Lenin's history. "Infantile" isn't an insult here but a clear statement about how such ideas are based in undeveloped thinking.