Subdomain is a part of the main domain name but can run another website with unique content. For example, you have a domain name : abcxyz123.com running a website.
If you want unique content to be displayed for a particular department such as HR, you can have hr.abcxyz123.com. The great thing about it, is that you don’t have to register another domain for adding a page about your HR.

Add-on domain is a unique domain that needs to be registered separately and has it own unique content. Changes in your nameservers are required for hosting the domain. In simple terms, its another website with emails running on a Multi domain hosting plan.
Parked domain is a unique domain that is registered which does not have a website ready yet. Parking the domain will basically forward it to the primary website hosted(on the hosting panel) as well as mask the primary domain name.