Server-side includes are a webserver feature that allows simple macro directives to be used in HTML files. They can be used to include other HTML files, set and displaying the contents of variables, and control what content appears on the page with `if` statements. They offer a simpler way of creating dynamic pages than a full scripting language like PHP or Perl.

This setting determines if new virtual servers will have SSI enabled after creation, disabled, or left to use the global default settings for your webserver.

For more information, see the Apache documentation at http://httpd.apache.org/docs/current/howto/ssi.html