Email alias is temporary address that doesnt have a physical email box and does not store any emails. It is used to redirect addresses to an actual email.
For example, test1@email1.com. It is currently forwarded to test2@email2.com. So whatever email is sent to test1@email1.com will arrive into the inbox of test2@email2.com. There is no actual email box test1@email1.com on your account which makes it an email alias.