SSI (an acronym for Server-Side Include) directives allow static HTML documents to be enhanced at run-time (e.g., when delivered to a client by Apache). The format of SSI directives is covered in the mod_include manual; suffice it to say that Apache supports not only SSI but xSSI (eXtended SSI) directives.
Processing a document at run-time is called parsing it; hence the term "parsed HTML" sometimes used for documents that contain SSI instructions. Parsing tends to be resource-consumptive compared to serving static files, and is not enabled by default. It can also interfere with the cachability of your documents, which can put a further load on your server. (See the next question for more information about this.)
To enable SSI processing, you need to
For additional information, see the Apache Week article on Using Server Side Includes.