The term root refers to the user name or account that by default has access to all commands and files on the server. It is also referred to as the root account, root user, and the superuser.
Root privileges are the powers that the root account has on the system. The root account is the most privileged on the system and has absolute power over it.
Here are 5 broad examples of what can be done within this account:
1. Edit whatever files the user chooses
2. Change the system however is desired
3. Provide and remove permissions to other accounts (which will affect whether the accounts can read, change, or execute certain directories and files)
4. Install software server-wide (MySQL, Apache, etc.)
5. Set up configurations for software throughout the server
For more information on terminology used in cPanel, check out this great resource: cPanel Glossary