Server side includes [SSI] are instructions embedded in HTML (as comments) which the web server processes when delivering a web page. Similar to dynamic web site technology (eg. ASP), but without programming capabilities.
Most frequently, SSIs can be used to share common content across multiple pages. This is ideal for standard headers and footers, or for common menu elements. I use SSIs extensively for this purpose (or equivalents in dynamic server side page generation eg. Cold Fusion).
A handy trick for controlling conditions which must be met to display content. The menu on this site checks the URL and displays the contents of the relevant section(s). eg. This page is in the experience section, so the items in the experience menu are displayed.
Two other simple but very useful possibilities with SSI are displaying the last modified date or file size.