Bandwidth is a term used to describe how much information can be transmitted over a connection. If you've ever wondered why it takes so long to download certain Web pages or other files to your computer, it's all determined by the bandwidth of the connection between your computer and your Internet Service Provider. Which is to say, it all hinges on that wire connecting your computer to your modem to your ISP (Internet Service Provider).
With respect to websites, the number of pages and size of images & videos and, how many times they're accessed by visitors affect bandwidth usage the most. In addition to website visitors, emails sent and received can also affect bandwidth usage.