Bandwidth is the amount of data that can be transferred to and from your account in one month. Everytime someone downloads a 10 kB file from your website, they use 10 kB of bandwidth from your account. This means every visitor to your site uses bandwidth, even if they just view the home page.
Bandwidth is reset on the 1st of every month.
If you want more bandwidth or disk space you must upgrade or order a second hosting package.