How much space and bandwidth you need.

[lemon_1800]

So basically bandwidth basically how fast the uploading of your site would be. And that's why if you have a lot of visitors that you need a higher bandwidth... Right?

Basically you are correct. Suppose if you are provided 45 GB bandwith, that means you can maximum access 45 GB of your web files at one time. If you get lots of visitors than the capacity of 45, your site will be down for a while and visitors get the message "it exceeded bandwith limit".