Bandwidth is the measure of data transfer or "traffic" that flows to transfer information to and from a web site via your computer. It can be measured by pure volume multiplied by the sizes of files that are included on your web page.
A web page may be very small or large depending upon the amount of text and the number and quality of images integrated within the web page. So if you have simple text on your web pages, small sized files are being transferred to the browser of the person viewing the page, and thus more traffic, at low bandwidth number is received. Web pages with images and videos will receive less traffic due to large size of files being transferred to the viewer's browser. How much bandwidth you actually require depends on the web hosting package. How much bandwidth is provided by the host depends on the price of the hosting package.
Since generally hosting plans have bandwidth requirements measured in months, you need to estimate the amount of bandwidth that will be required by your site on a monthly basis. Generally hosting companies charge you with an over usage fees if you go above the bandwidth that came with the package. If you know what you are doing with your website and what the purpose of your site is, you should be able to predict how much bandwidth you would be needing. For a beginner, it is best to initiate with a very basic web hosting package with little emphasis on bandwidth. As you start gaining traffic slowly and have an idea about bandwidth, you can then bargain with your host for a high bandwidth package.