I've heard the best way to tackle the Quake codebase was to pick a 'component', such as the renderer, and start to understand how that works. Work your way out, one component at a time. Eventually the bigger picture comes together. The same would probably work for this, though unless you're well versed in game and graphics programming nomenclature some of it will go over your head, which is expected - when you see something like that, think of it as an opportunity to learn a new concept and look it up.