Bandwidth is the amount of data transferred to and from the webserver. We measure the amount of data transferred within one calendar month.
If an image is 100kb and is downloaded 1000 times in a calendar month, that would be 100,000kb (or 100mb) of bandwidth. If you have a 1mb site and it's viewed 50,000 times, that would be 50,000 mb (or 50gb) bandwidth. Obviously it can be quite hard to work out how big your site is and also how many times it will be viewed.
Here are some rough estimations:
A small blog: 5GB bandwidth
A photo gallery: 25GB bandwidth
A forum: 100GB bandwidth
An ecommerce store: 50GB bandwidth
A busy site: 200GB bandwidth