An alias is an additional email address which delivers mail to an existing email address. For example, you can give an employee, John (email@example.com) the aliases firstname.lastname@example.org and email@example.com. Messages sent to these addresses will go straight to John's inbox. This allows you to send and receive email from several different email addresses, but manage them from one inbox.
Each alias you create counts as one email address in your total allotment. For this reason, this feature is not available to Custom Mailbox users.