The Grundrisse is Marx's rough draft of his economic theories - of much of what was later rewritten and published as Capital, and more - written at high speed in 1857-8.
Because it's a rough draft, Marx lets his mind roam more freely, and pursues his lines of argument further into a speculative future, than in his finished works. The Grundrisse can give us pointers to understand the capitalism of microelectronics, globalisation, and privatisation - in which some of the inherent tendencies of capitalism are working themselves out further and more thoroughly than even seemed possible in Marx's day.
In the Grundrisse, we also read Marx writing more directly and freshly about some of his basic ideas than in his finished works, giving us a better idea of how he reached his conclusions.
Of course, there are also obscure and difficult bits. Our plan is not to plod through a detailed discussion of every sentence or every page in the text. Some of us will study the full text in detail; others can take part, very usefully, while reading only short excerpts.