

It can mean several things. Here we are using it to insert text from one file into another file automatically when that file is sent to the browser. A common use would be to have a standard footer that is automatically appended to multiple pages. It doesn't HAVE to be a footer. You can insert at the top, in the middle, anywhere.
Note: The file includes the Server Side Include must have an extension of .html.
<!--#include file="footer.html"-->
or
<!--#include
file="includes/footer.html"-->
You don't have to put the included file in the same directory or subdirectory of the same directory. You can refer to files anywhere, but the syntax is slightly different:
<!--#include virtual="/~coleman/includes/footer.html"
-->
Notice that this is the same as the path you would type into a browser,
minus the http://www.library.yale.edu.
<!--#flastmod
virtual="/~lso/workstation/docs/ssi/example.html" -->
include code. That's because the server
"creates" the file for you each time you request it.