Each time someone visits your website, their computers will download information from your website host. The more people visit your website, the more information that will be downloaded from your website servers.
Website bandwidth refers to the total amount of data that can be transferred between your website hosting and your users on a given month. So if your website bandwidth is 50GB, that means that up to 50GB of information can be transferred each month to all your users. Typically someone visiting your website will download about 1-2MB of data via normal browsing.