How do you go about understanding other (open source) projects?

I can easily understand single file projects or projects with just three or four files, but I'm trying to understand a bigger project (insight3d). I get that I have to understand the general flow of the program before looking at the individual functions, but I spend almost all my time just looking through the roughly 100 files for functions so that I can see where the program goes next or to identify which functions are helper functions. I don't think it's supposed to be this complicated. How do you guys get around these larger projects? Is there some piece of software I should have?

Thanks