An append structure is an additional structure you can apply to tables and database structures to extend their field lists.
This lets you add additional fields to a standard SAP table or structure without modifying it.
Generally this is done to extend standard functionality using userexits etc.
So I guess an append structure in a smart form is appending a table or structure that is used in a smartform so that you can add additional functionality to a smartform. For example to print additional fields not supplied by the smartform.
Hope that helps.