The terms Bandwidth and Data Transfer are frequently used interchangeably which can be misleading. Bandwidth refers to the total amount of data that can theoretically be transferred at any one time. Data Transfer refers to the amount of data that has actually been sent from the server to the client or vice versa. An analogy would be a road, the number of lanes on a road would represent Bandwidth, the number of cars that travel along that road over a given period of time would represent Data Transfer.
Everytime a user opens a page of your website the data is transferred from the server to the user i.e. the users web browser. Typically, a page with a few graphics (GIFs or JPEGs) will probably require the transfer of about 50,000 bytes of data to your browser.
If you exceed your Monthly Data Transfer limit and have not purchased any Pre Paid Data Transfer then your website will be unavailable.
All of the hosting packages are allocated a Monthly Data Transfer limit. The graph at the top of the page allows you to keep track of how much of your Monthly Data Transfer allocation you have used.