By clicking on the Resolve Alias link a new window will appear the option to enter an alias. This does an alias lookup to find the final destination of an alias.
For example, if you have a user webmaster, webmaster may have an alias to www. So that mail to www@domain.com will go to webmaster. Additionally you may have another alias, web which points to the www alias. So if someone sends an e-mail to web it will track through the aliases and ultimately be delivered to the webmaster account.