Your Hub Score simply illustrates in numerical form how actively involved you are here on Hub Pages. It is made up of a combination of factors such as how many hubs you post, how many comments you have/leave, how many questions you ask/answer, etc.
It will fluctuate as you do more or less of these things. In the grand scheme of things (earnings, personal enjoyment, etc.) it doesn't really reflect anything significant.
You can read more about it from the horse's mouth here: https://hubpages.com/faq/#hubscore