The hub scores are an overall indication of how good a hub is. It fluctuates based on how it is reviewed and interacted with over time. There are a variety of characteristics about your hubs that determine what kind of hub score it can achieve, like the content of the hub, length of the hub, amount of traffic the hub receives, etc. Your hubber score (the number in your thumbnail picture) can also influence your hub scores. Basically, positive events to your hubpages profile and hubs will increase your hubs' scores. Similarly, negative events will have the opposite effect. As for how the scores are actually calculated, HubPages is keeping that private.
The scores are just indicators for how your hubs and collective existence on HubPages are doing. They don't really have a bearing on anything other than informing you.
Check out the link below for more info. Hope this helps! :)