An addon domain is a feature of a cPanel account where you can host more than one website on that specific cPanel account. cPanel is the interface that you will see when you log in to a hosting account.
How to setup an addon domain
LOGIN into your CPANEL account
In the section DOMAINS you will find Addon DOMAINS
Enter the new domain name
Subdomain will be automatically created as the new document root
Please also select the FTP option if you need it