Let's say your domain is domain.tld, and you want to install a message board. There are generally 2 options on how to make it visible:
board.domain.tld and domain.tld/board
The second option can be done with the help of subdomains - check this article on how to create them.
When activating a hosting account, you used a domain name. It's your main domain name.
An add-on domain is a domain, added to the existing hosting account, which shares the same resources as the main domain and other add-ons. It has a separate name, for example, while your main domain is domain.tld, your add-on can be website.tld.
When to use add-on domains?
If you want to create a new website, not related to an already existing one
If you want to create a redirect from one domain to another
If this way of adding a domain suits you, check this article on how to create an add-on domain.
A parked domain is a domain, which shows exactly the same content as another domain (to which this domain is parked). Same as an add-on domain, it can be a completely different domain name. A parked domain can be also called a "mirror".
When to park a domain?
If you want one website to have several addresses, and changes made on one of them to be reflected on the other one momentarily, then you need to park a domain.
What is the difference between redirect and parking?
If your website is domain.tld:
If you add an add-on domain website.tld, and create a redirect to domain.tld, then the visitor, when looking for "website.tld", will be fully redirected to a different address (and see this address in the browser)
If you park domain website.tld to domain.tld, then visitor of website.tld will see exactly the same website as domain.tld (with all the latest changes), but in address bar they will see website.tld
If parking suits your needs, make sure to check this article on how to park a domain.
Both parking domain and adding domain as an add-on require you to purchase this new domain name