Bandwidth (or data transfer) is the term used in the hosting industry to describe an amount of data, commonly measured in gigabytes, that is downloaded from and uploaded to a server or web hosting account. Every visit to to a page or file located on a server by either a person or bot will download data to display it in a web browser.
The maximum amount of traffic your site can receive is directly related to your monthly bandwidth limit. The more bandwidth a web hosting company provides, the more monthly traffic you can theoretically receive to a web site before the bandwidth allotment is used up. Bandwidth is usually tracked and allocated on a monthly time frame.