It's pretty simple to do. While you're sifting through the results of a Google search, there will now be a link you can click to block the website from future searches. Once you click it, you won't see it again.
You have to be logged into your Google account to use the feature. Your blocked sites can be managed by going into your Search Settings and going into the "Manage blocked sites" section.
Honestly, unless you have kids or get easily offended, we really don't see this being a big deal for most people. Google claims the blocking of certain websites don't affect their ranking, but it may in the future. So be careful who you block. It's like Spider-Man said, I'm walking on sunshine! with great power, comes great responsibility.
[Mashablevia Google Blog]