Addon domains allow you to control multiple domains from a single account. An addon domain links a new domain name to a directory in your account, and then stores its files in that directory.
To access Addon Domains, go to the DOMAINS area of your cPanel.
From here you have the ability to create a new domains or edit the settings of an existing one. The website root will be a newly created folder inside of your public_html.
After the addon domain is created, you have the ability to change what folder the document root folder that was set when the domain was created. You also have the ability to override that and have the addon domain function as a redirect to another URL using the manage redirect function.