Bandwidth is the amount of data that can be transferred to and from your hosting account in one month. Everytime someone downloads a image or views a page with lots of images (.i.e 4MB in size) from your website, they use 4 MB of bandwidth from your account which will be recorded in your bandwidth usage statistics.
Bandwidth is reset on the 1st of every month and the usage is updated every day.
If you want more bandwidth or disk space than what your present hosting plan offers, you can upgrade or buy more bandwidth depending on plan offered.