I looked at that link, I didn't realize that was a way to use "AS". Anyway, you probably figured out already that it's there (I think) to avoid having to put TABLE_PREFIX in everywhere the table name is used.
Of course this is a reference and not a tutorial, but in case you missed the link I added above, you might find this useful:
http://dev.mysql.com/doc/refman/5.4/en/sql-syntax.html