The number of links from other places on the web that point to your Hub are represented as a rough average by the number of stars shown. The more stars, the more incoming links. They are also known as backlinks.
When people like what you wrote and feel it can be useful to their audience, they may link to your Hub. That happens slowly over time and helps build your search engine ranking. However, if you try to create your own backlinks, it can actually hurt your ranking because Google knows what's real.