SSI is a system that among other features enables you to include or encapsulate an HTML file in another HTML file. This is very practical if you e.g. want the same menu on several pages without having to copy the same HTML code to each and every file. For the server to search for and recognize SSI directives in your HTML file, you will need Web hosting Medium or larger. The file must end in ".shtml" instead of ".html" or ".htm". For more info about features in SSI, see here: