Anti-sec Movement

Could anyone tell me what the anti-sec movement is? And what is their purpose?

ImageShack, one of the web’s largest image hosts, was hacked by a movement called “Anti-Sec” recently. I still cannot figure out what the anti-sec movement is but I did know that it damaged my website. Thanks to it, I have to upload some of my picture again. The hacker replaced most pictures of ImageShack with the following message.