An alias domain is used to redirect a domain that you do not have an active website for, to a domain that does. You can use your cPanel for managing all your alias domains.
For example, you may have purchased several tlds of your domain name such as example.com, example.org, and example.ninja (it’s real, trust me). Instead of using your addon domains for these extra tlds, you can set them up as aliases of the primary domain. This way, any traffic for the alias domains will be redirected to your main domain website.
Here is a guide to help with adding managing your alias domains.