Disk space used includes the contents of the home directory, e.g. website files, images, databases, any emails stored, any backups stored (webmasters should really store these elswhere), Logs, Caches, administrative files and any databases.
You can check the disk space used by your website and other files in cPanel: Files: Disk space used. Then you can delete unwanted files using File Manager.
Over time Logs and Caches accumulate, taking up disk space. You can delete the contents of both folders to allow disk space and save from upgrading to the next hosting plan.
The Tmp folder contains all web statistics. Usually only the current year for AWStats and Visitors is read, so can old stats can be deleted.
Emails are often left on the server because the user forgets to change the settings on their computer to delete them from the server. In that case these can be deleted to give more space.
Images have often been loaded straight from a camera. Their camera size may be 2 to 4 Mb but the web page may only need them at 100 Kb. WordPress optimizes the images before publishing each page but stores the original image at full size. These can be deleted to provide more space.
Using only the space you need can qualify you for the right hosting plan.