This means that stories with a tag that you are following will appear in your hubfeed. For example, if you follow #space, all submissions with the #space tag will appear in your hubfeed, regardless of the author. Tags can be followed simply by clicking 'follow #tag' on that tag's page. Currently, a tag's page is hubski.com/tag?id=thetag In the future, I'll be looking to change this to something shorter like hubski.com/t/thetag
As an aside, you may have noticed that until recently, tags were called topics. The change was made for a couple of reasons, however their purpose remains the same.
As always, feel free to share feedback and ideas.