SSI (Server Side Includes) are directives that are placed in pages and evaluated on the server while the pages are being served. They let you add dynamically generated content to an existing page, without having to serve the entire page via a CGI program, or other dynamic technology.
The decision of when to use SSI, and when to have your page entirely generated by some program, is usually a matter of how much of the page is static, and how much needs to be recalculated every time the page is served. SSI is a great way to add small pieces of information, such as the current time. But if a majority of your page is being generated at the time that it is served, you need to look for some other solution.
Server side includes also allow you to use common website elements on every page of your websites such as menu's, page footers, headers or adverts and gives you the ability to make rapid site-wide updates to just 1 file, such as a website main menu or common page footers.