I don't understand php GD

Hi,

I have been a fan of this plug in for a while. But it does not seem to be working on my site. I have the the current version of wordpress and of the plug in.

I am getting a broken image signal with this plugin.

Could someone please explain to me simply what the PHP GD is and how to check if it is enabled/installed.

Thanks.

http://todaysbighit.com

http://ww.wp.xz.cn/extend/plugins/wordpress-popular-posts/