[Flankk]

What you need, more than anything, is the mental model the authors used when writing it. In a company you want to sit down with the people who wrote it. If it's open source you'll have to do the same if the API is undocumented. Otherwise it's something like unbaking a cake. I've done it myself by first figuring out the dependency tree of the modules, then trying to get a high-level idea of the purpose of each component. From there, it should become easier. If the code is written with any sense at all it will get lower-level and less abstract as you work your way down to the function calls.