GZip is a compression format that webpages can use. It is similar to ZIP. Basically, your webpage can send a "zipped" up file to a browser in order to make the file size smaller, and then your browser can "unzip" it and display it. This is real usefull when you are trying to send big pages to people on slow connections.
If you want to impliment it, this is a real good class that works on most all configurations.
http://leknor.com/code/php/view/class.gzip_encode.php.txt
To Impliment it, all you do is include the above code at the TOP of the page you are trying to display like this:
<?php
ob_start ("ob_gzhandler");
include('gzip.php');
?>
This is assuming that you save the code from the link that I gave you as gzip.php.