An alias domain is a domain that shows the same content as the domain it is an alias for.
For example, if you have a website at website.com, with an alias domain, you can display the same website at aliasdomain.com
To create an alias domain, you need to go to cPanel -> Domains
Here, fill in your desired alias domain and make sure that it is ticked in the part-document root, as shown below:
This will create an alias for your main domain using the "public_html" folder.
Alias to an add-on domain?
If you want to create an alias for an add-on domain, you should untick in "Share document root" and then specify the folder name for the domain you want to create an alias for.
For example, if you have website2.com, just write this under folder name and nothing else (no / or similar in front)