Every photo, video and file you keep online has to live somewhere. That's data hosting — storage capacity you rent instead of buying and maintaining hardware yourself.
Bandwidth is the other half of the equation: it's the amount of data moving in and out of that storage. Uploading a backup uses bandwidth. So does every person who downloads or streams something you've stored.
The two are billed differently almost everywhere, which is part of why cloud bills get confusing. We keep both at the same flat rate so there's one number to remember, not two.