Simply put, it's data transferring to and from the site when users hit or upload to your site. Whenever someone hits your site, it sends data to their computer screen, which causes bandwidth transfer. This is one of the main things you pay for with hosting alongside of storage. The average full site is probably is about 50 mb or less, so it takes many thousands of "hits" to your website to transfer the amount of bandwidth we give even in our first plan. We give 30 GB in our first plan.
Just to understand about bytes...
1000 KB = 1 MB
1000 MB = 1 GB
1000 GB = 1 TB
