It relates to the number of hot links to other hub pages, including your own, that are embedded within your articles.
When you look at a hub you've written, at the top you will see as "Suggest Links" tab. Click that and possibly relevant links in your article will be highlighted. When you click on them, you get a list of other articles that might relate to expand the impact of your articles. Some are ridiculously irrelevant, but a good number work, and it's a good way to draw readers to your other hubs as well of those of hubbers you like. I enjoy setting up a link to things that look like they might be intentionally funny or in contrast. I often have a link to Gary Larson's Far Side cartoons, and I frequently link it as a treat to readers.
Including links doesn't do much to improve your scores, unless they keep readers reading, but they certainly increase your exposure and readership.
I hope this is helpful.