What are Server Side Includes (SSI)?
SSI (Server Side Includes) are instructions that are placed in HTML pages, and processed on the server while the pages are being served. They let you add dynamically generated content to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology.
They can be used to add a small section of data like date or time to an existing page but are not suitable for generating the entire page as it is served.
In fact they have been virtually replaced by javascript and PHP. I personally don't recall seeing a site using them for the last couple of years.