With an alias domain you can have a second email address where you can receive messages. When you create one, all the Mailboxes and Email aliases of the original domain are present in the alias domain.
You can create a new alias domain simply clicking on the number of them in the domains table.
And click on the plus button at the top of the Alias Domain page
You will be asked to enter the alias for the domain, and after that, you need to verify the ownership of it before it can be active..