This change was designed to punish pages that are "spamming Google".
This includes pages that have little content, built in quantity merely to pass an anchor link, pages with keyword stuffing and cloaking that have always been bad and of course, hidden text, thin sites...the list goes on.
Luckily I knew about this a while ago and posted here and changed the way I do things so it helped my sites.
We are starting to hear the stories in the forum so read up on what you can do
Read more here