The number is your hub score. It is made up of points you receive for your hubs and/or your answers to questions.
Your hubs are voted either up or down according to how the reader feels about it. If your hub is voted up, your score goes up; if your hub is voted down, your score goes down. The same is true of any answers you give.
In order to keep your hub score moving up, you must post hubs and/or answer questions (as I'm doing now) that have some value to the reader.
Using this answer as an example - I have gained one point by answering this question. If people wo read it agree with what I have written and vote me up, my score will rise. Of course, there is the chance that someone will disagree and give me a negative vote which will bring my hub score down.
Keep publishing quality material and your score will rise.