See Hubscore and Hubber Score written by SimoneSmith, a HubPages employee, for the answer to your question:
Basically, Hub score measures a number of factors, INCLUDING traffic to each hub. It also has some way of evaluating where the traffic comes from. As Simone says, "The quality of your traffic will also influence the metric (if a higher percentage of your traffic comes from reputable sources, you'll see a more positive effect on your HubScore)."
Hubber score takes into the average hubscores of all your hubs, then ALSO factors in how you participate in the Hubpages community.
Note to michiganman (since comments aren't working): If your hub's score is below 40, its links become nofollow. If your hubber (author) score falls below 75, all your links become nofollow.
A really low Hubscore can cause a hub to lose Google traffic, because hubs with scores below 40 are (I think?) also not featured in "related" links in the sidebar of other hubs or listed in the Hubpages directory of hubs. Google finds hubs through links, so if those links aren't there, Google may not send traffic.
But usually, it's the other way around: if your hub gets at least some traffic, its hubscore goes up, and lots of hubs getting traffic make your overall hubber score go up.