100 MB bandwith means people can download 100Mbs only from your site. (THIS INCLUDES THE WEBPAGES and images)

What this means is, that every time someone logs on your site, they download perhaps 20 kbs on the page and 200 kbs on the images .. those count on a monthly 100MBS limit. If you run out of bandwith, your site simply does not load -unless you buy more off course-.