Wow, there's soi much info on Google Panda on Hubpages already because of the impact it had on hubpages.
It was an update to the Google search algorithm which included an attempt to weed out low quality editorial content, especially duplicate content and content farms. That hit Hubpages and to a lesser extent Squidoo because people use them for SEO purposes (which is legit) by copying in poor content (which isn't).
It came out Feb 2011, and since then Hubpages has implemented a set of changes to ensure content is better quality, and most people think the traffic has improved again (based on hubs talking about it - I wasn't on Hubpages when Panda hit).
More info on wikipedia: http://en.wikipedia.org/wiki/Google_Panda