Email alias is not a real email account. Instead it's an address that forwards all email it receives to another email account. For example, if you establish an email alias called info@yourdomain.com to go to johndoe@yourdomain.com, then all email sent to info@yourdomain.com will be automatically forwarded to johndoe@yourodmain.com (In this case, 'info' is called an alias of 'johndoe'.)
You can setup email alias so that email is forwarded to an account outside of your domain name as well. For example, you can setup info@yourdomain.com to go to johndoe@aol.com.
You can also setup email alias to forward to multiple usernames or another alias.
If an alias is set up the same name as a username, the alias will override the username.
This way, you can have unlimited email addresses to fit your needs with only a handful of actual email accounts.