You are going to keep on encountering this as you read up on theory and history.

Try not to be too down on yourself when it happens because you shouldn’t expect yourself to be completely across the most important topics of political debate in Europe from nearly two centuries ago.

Imagine a person in a century from now reading articles from, say, The Guardian and coming across something which references “Trumpism” or “the MAGA movement” to critique it; that person almost certainly isn’t going to understand what the MAGA movement refers to but the Guardian article is going to treat it as if everyone grasps what they’re referring to because the Guardian is part of a contemporary discussion right in this very moment where it’s topical and relevant and so of course everyone grasps what it means today but this will not be the case in a hundred year’s time.

I’d recommend one of two approaches here:

Either skip over these sorts of terms because the fact that they don’t mean anything to you may be indicative of the fact that they are no longer relevant to contemporary politics (for example, you don’t hear people talking about Manichaeism or Fabianism today because it bears no relevance to today’s politics) or to put a little note next to the name with a shorthand version of what that person’s thought represents (for example, when reading Lenin lambast Bernstein you might put a little note saying “incremental reformism under bourgeois democracy to achieve socialism” so that whenever you encounter Lenin striking out against Bernsteinism then you can know what he’s really criticising when he does it.)

It will make more sense as you read more theory. Good luck with it!