An alias is a handy way of creating multiple usernames without setting up user accounts.
Let's say there are 2 users in the mail account, admin and John.
By default, 'admin' would have an alias already setup to collect any emails that were not addressed to 'admin@' or 'john@'. So if an email was sent to 'sales@' it would be redirected to 'admin@'.
This is a basic alias but a good idea how it works.
Aliases are also useful where someones name may have different spellings. In John's case, his name could be spelt 'jon@' or 'jonathan@' or even 'jonny@'. By default these incorrect spellings would be sent to the 'admin@' mailbox. This could be inconvenient if the 2 established mailboxes are used by different people.
So, in webmail you have the option of creating aliases. In this example new aliases would need to be created for 'jon@', 'jonathan@' and possible 'jonny@'. These aliases would be told to direct to the 'john@' mailbox.
If John was promoted to sales, a new alias 'sales@' could be created pointing to his mailbox.
To create an alias. Login to your webmail using your admin user. Go to 'options and styles' at the top right of the screen and go to the admin section.