An email alias is an email address that doesn't have a mailbox. It forwards on any email sent to it to another address that was setup by the administrator. It does not have a username and password associated with it.
An example:
My email alias is jdoe11@indstate.edu.
It is setup to forward to my OWA account jdoe11@mail.indstate.edu.
My friend sends an email to jdoe11@indstate.edu and it arrives in my Outlook Web Access inbox at jdoe11@mail.indstate.edu a few minutes later.