What does bandwidth refer to?
The term Bandwidth refers to how much data transferred between our servers (where your website is located). For example, in the Free Website Hosting service we allow your account to transfer 1500 MB of data each month.
Data refers to all of the files on your website... images, webpages, everything.
Every file on your website has a specific filesize. Let's say you have a webpage with a filesize of 12kb, and you have 3 images on that page which each have a filesize of 20kb (3x20kb = 60kb) and a midi sound file, that plays automatically on your page, with a filesize of 28kb, that is a total of 100kb transferred to your visitors browser when they load your page.
When that 100kb page is loaded 10 times into visitors browsers, that uses up 1,000kb (10x100kb = 1,000kb) or 1MB of bandwidth.
Everything that is transferred contributes to your bandwidth usage, including uploading files to your website.