This does exactly what it says, it includes the files on the server end. So by the time the file gets to the user's browser, it's all one file, the user never sees that it has been pulled for multiple files.
You can use SSI for something as large as storing your websites entire template, or something as small as your copywrite lines for easy updating every year.
You can include a file within your PHP file using this code:
INCLUDE 'http://www.yoursite.com/path/to/file.php' ;
Of course you would change it to the actual URL and path of your file. You can included as many files as you want, you are not limited to one include per file.

