There are a lot of code in open source projects, looking at all of the code is time consuming and can be confusing to a novice like me. Are there any sections of open-source projects that should be focused on?

What should I focus on when I look at code?

I'm asking this in general because if I ask this specifically, the question will only apply in one or two projects rather than an entire group of projects ranging in different types of games and difficulty.