SSI stands for Server Side Include. Its another way of using frames, just a whole lot easier! Some sites use the same layout on every page and webmasters get tired of writing the same HTML over and over! So SSI was created to stop that. If you have a nav bar. Like on the left of my site, and it was called left.html, SSI can be used to include that in the pages that you want,
<---#include virtual="left.html"--->
This will have the left.html whereever you told it to be! you can do this for any thing you like. Just substitute left for anything else!
REMEMBER! ALWAYS have a .shtml extension, or it wont work!