An addon domain is a separate domain that is hosted on your primary domain but it appears as if it is hosted on it's own account. If you were to addon "newdomain.com", a user browsing to your site will see "newdomain.com" in the URL, but it will be redirecting it to a specific folder under your primary domain.
To addon a domain please follow these steps:
Firstly please ensure that the name servers are pointing to right name server
Login to your cPanel account and select 'Addon Domains' under the 'Domains' menu.
Complete the form as follows:
New Domain Name: the domain you wish to add
Subdomain/FTP Username: Whatever you put in the this field will be the folder that is created under your primary account. It will also be the default username for mail and FTP.
Password: your password
The change in nameservers may take up to 48 hours.