For tables, an Alias can be thought of as a virtual copy. "MyTable AS XXX" means that XXX now refers to a copy of Mytable. It's as simple as that.
An Alias for a field can be thought of as a virtual field. "MyField AS NewName" means that NewName refers to MyField. "(3 * MyField) As TripleMyField" means that TripleMyField refers to the product of MyField and 3.