Normally, when you request a page from a server, the server sends the contents of the file just as they are stored. The server doesn't look at the contents of the file. Each file is self-contained and static. (The browser sends separate requests for inline images and other inline links.)
However, you can make the server look at the contents of the file and include information like the contents of another file, a value of an environment variable, the date the file was last modified, or the output of a program within the page. This capability is called "server-parsed HTML". You may also see it called "server-side includes" or "SSI".
Server-parsed HTML is useful for two reasons:
The trade off for these benefits is that parsing your pages takes extra time. The server may be noticeably slower if your pages are long. If you want to include a standard trailer to the end of all your pages, you can use trailers (page 9), which are much faster than server-parsed HTML.