SSI or Server Side Includes, is actually a sort of server-side programming language interpreted by Nginx. Its name originates from the fact that the most used functionality of the language is the include command. Back in the 1990s, such languages were employed in order to render web pages dynamically, from simple static .html files with client-side scripts to complex pages with server-processed instructions. Within the HTML source code, webmasters could now insert server-interpreted directives, which would then lead the way to much more advanced pre-processors, such as PHP or ASP.
The most famous illustration of SSI is the quote of the day example. In order to insert a new quote every day at the top of each page of their website, webmasters would have to edit out the HTML source of every page of the site, updating the old quote manually. With Server Side Includes, a single command suffices to simplify the task: