If you're a programmer, you most likely already know GitHub. How active are you? Do you have an account? Have you explored all of its features and possibilities? It's common practice for recruiters to research talent on sites like GitHub and Stackoverflow because it gives them a chance to truly understand your expertise and knowledge in your discipline.
If GitHub is something you want to learn more about, start with this video from Lynda.com. It takes you through Git basics, GitHub basics, the GitHub Client, and how to collaborate with GitHub.