Addon, Parked & Primary domain are terms used when managing multiple domain names, within the same hosting control panel. They denote the configuration of a domain name, server-side through your Hosting Control Panel.
Primary domain (Main): This is the main domain name of your hosting plan/account. This is the first chosen domain to be setup on your hosting space.
Add-on domain (Additional): This is an additional website in your hosting plan. This is a full separate website, with domain name, website content, email accounts within the same hosting space & control panel. Example: Two domain names say company-domain.net & my-domain.com, setup with company-name.com being the Primary domain and my-domain.com as Addon domain. In this setup, both domains will appear as complete separate websites to visitors. They are both hosted in the same plan and controlled via a single control panel interface.
Parked domain (Alias) : This is an additional domain name to browse an existing website. Parked domain points to an existing website. Parked domain's are used when you want to have multiple domain-names to display the same website. Example: company-name.com, brand-name.com and product-name.net showing the same data/content. Company-name.com is Primary domain while brand-name.com and product-name.net are Parked domains