The amount of hard disk space your files take up multiplied by the amount of people that access them each month equals your bandwidth usage.
The amount of disk space and bandwidth you will need to host your site(s) can depend on a variety of factors. If you plan to host a lot of big downloads or maybe a bunch of videos, audio clips or pictures, then all of these can quickly consume your space and bandwidth.
When you’re first starting in the world of web hosting, it’s probably safe to go for a plan with average storage and transfer limits.