GD is an image library from PHP.
Most all servers have GD enabled (Do you have Captcha on your forum? then you probably have it installed), but that variable is there in case you don't have it so you can turn it off and not get errors.
I use your forum to resize images on the fly so that they are not huge when you download them to your phone.
To Enable GD:
http://php.net/manual/en/image.installation.php
Or have your server administrator do it.