"Bandwidth" is the total size of all files uploaded and  downloaded, for a month.

This means, if many people come to your site, you will have higher bandwidth. This is good.

But it also means if you upload large files often to your site, you will also have higher bandwith.

Rule of thumb: The average high-use Tibetan websites i know of, use around 10,000 MB (10GB) of bandwidth per month.

Note: "Unlimited" just means, no limit unless you get so big that traffice to your site is putting stress on the server! If that happens, you will be encouraged to upgrade. (And if that happens, your site has become really popular ... so it's a good thing, not a problem!)