Include codes are the codes used by the system to call in various info to populate your page. Those can be the menu, the main content, the site message, the meta tags, the page/product name ( title ), product information...etc. All are stored in the database and called into your page using the includes.
A listing of codes available is in your storeadmin "Template" area. Click
Templates, then within the body of the text in the
Custom Template box there is a link '
site include codes' . You can also find a listing of includes, with the required includes higlighted,
here and
here ( for version 2.71 )
