This document is assembled from several others using server side includes. In this case, the files doctype.html, header.html, and footer.html are added to this file (ssi.shtml) by the web server. The result is a fully developed page.
To enable server side includes, make sure your file ends in .shtml. Then add the includes and other code.
The basic syntax: <!--#include virtual="filename.html"-->
That will add the contents of filename.html to the current document.
If you are adding files in the same directory, you only need the filename, not directory information. You can get files from a different directory in a few ways. To go up one level, try:
<!--#include virtual="../filename.html"-->
Or use absolute URLs, like this:
<!--#include virtual="/AX-Student/filename.html"-->
That way, you can keep a bunch of files in your top level web directory and use them from all over your site, which is very efficient.