your question is too broad to be answered, but I'll give you a hint of what I mean.
Say you check out the source code of Jetty, the web server. Or Tomcat. Or the source code of Wicket, the web framework. Or Git. Or whatever really.
Then it helps to have a specific use case in mind ? "How does a textfield get rendered". "How does Jetty parse HTTP headers?" "what does GIT do when you call "commit"". Of course, these could be huge topics, but it doesn't really matter, because you don't need to get a 10000% understanding. It's more about the general feel of how people solve things, the patterns, the coding style. Especially coding style wise, you'll learn that many open source projects have completely opposite rules, when it comes to coding styles. And it really often only is a matter of preferences and homogenity , instead of blind believe in certain coding rules ;)