See this page in the FAQ's http://hubpages.com/faq/#hubscorecalc
It basically lists these things:
Amount of traffic - including percentage from reputable sources other than HubPages
The reputation of the Hubber - your Hubber Score and contribution to the community
The response of readers to your hub - including comments, thumbs up, etc.
The uniqueness of content - copying content already available on the web will be penalized