From Apache manual: SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated 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.
SSI is useful when you have a piece of HTML code that is massively duplicated in all your HTML files, like navigation headers. To simplify development, you can move that piece of common HTML code to a separate file, then use SSI on other HTML files to include it.
To use SSI, the only requirement is that you need to name your HTML files with the extension .shtml.
Note
SSI is only useful for simple includes, for creating more complex web sites, you need to use full programming languages like PHP or Perl.
For more information about SSI usage, please see the Apache web server SSI documentation.