The "Add On Domain" option in your cPanel lets you add extra domains as add-on domains to your hosting account.
This lets you host additional domains from your account.
1: Login to your cPanel which can be found at your domain name forward slash cPanel, for example: http://www.yourdomain.co.za/cpanel
2: Next scroll down to the section titled Domains, and click on Add-on Domains
3: Fill in your new domain name with the extension
4: Insert a password for this domain, this password will be used for the domains FTP access.
5: Click Add Domain and your new domain will have been added to your cPanel.
You can upload files directly to this add-on domain using the username and password for the add-on domain to log in via FTP.