It stands for "Server-side includes" and it's main use is for including pieces of a HTML document on the server before it gets sent to a visitor's browser.
Take this for example: a website can have a navigation bar that sometimes changes. But what if there are thousands of pages? Then if you wanted to change the navbar, you would have to edit every single page! But with server-side includes, you would need to change only one file to update the whole site.
Here's what an SSI tag might look like:
Code:
<!--#include file="navigation.txt" -->
If navigation.txt contains the code for the navbar, then if you change it, all the other pages that have that SSI tag will "magically" update.
For more information, check out some tutorials like
this one.