Ask HN: Github as resume, what does it really mean?

It is so often said "your github account can be a great resume", but what does that really mean? What will a potentional employer specifically look at on your github account? Will they glance at the most recent project you committed to and assume this reflects the majority of your work? What if you have forks of projects that you have not actually committed any pull requests to at all?

If you were an employer, what would you look at on a github account?