Critical CSS?

When I run the Hummingbird test, I get a message telling me to Eliminate render-blocking resources. It then goes on to tell me to combine files and to move non critical files to the footer. My question is — how do I know which files can be combined and which are critical.non-critical? I also see some code and it talks about inline css options which I have no idea how to find or use. Please take pity on a confused noob. Thanks