Ask Lemmygrad

That can happen with theory. Are you wondering who Sismondi is? You're going to keep coming across this problem, I'm afraid. I'd say just read it quite quickly and focus on the bits you do understand. Then read something else and do the same. Then come back to the Manifesto later.

You don't have to understand every word/section the first time you read anything although I understand the urge to try. I think part of the trick with any kind of learning is letting go of the idea that you have to understand everything the first time. I think schools/universities teach this way impliedly and explicitly and it's a travesty. It slows you down over potentially insignificant details.

If you just keep going and read things multiple times, you get a feel for what's important because you'll see the same thing discussed elsewhere. Chances are if no other Marxist has picked up an argument since Marx, it's mainly of academic interest. The crucial bits appear again and again. (Not to say there's no value in reading carefully or analysing lesser known ideas but save it for later when you've got more of a foothold.)

From Chris Harman's, 'How Marxism Works':

But though Marxism is not difficult, there is a problem for the reader who comes across Marx’s writings for the first time. Marx wrote well over a century ago. He used the language of the time, complete with references to individuals and events then familiar to virtually everyone, now known only to specialist historians.

I remember my own bafflement when, while still at school, I tried to read his pamphlet The 18th Brumaire of Louis Bonaparte.

I didn’t know either what Brumaire was or who Louis Bonaparte was. How many socialists have abandoned attempts to come to grips with Marxism after such experiences!

This is the justification for this short book. It seeks to provide an introduction to Marxist ideas, which will make it easier for socialists to follow what Marx was on about and to understand the development of Marxism since then in the hands of Frederick Engels, Rosa Luxemburg, Vladimir Lenin, Leon Trotsky, and a whole host of lesser thinkers.

Maybe this pamphlet is what you're looking for? (Link above)

One thing I'd recommend is Engles, Socialism: Scientific and Utopian. This will give you some explanation of the ideas of people from around the time of Marx and Engels. But in a way it will be more of the same as Engels will introduce loads of new names.

PS I can't tell you about Sismondi. I glossed over it and have yet to get round to looking further into it.