Google Social Search is not a new feature, but it wasn't that important until now. Google used to display at the bottom of the search results page a few links to pages created or recommended by your friends and social connections. The feature automatically obtained data from Google Reader, Google Buzz, Gmail Contacts, Twitter and other sites linked from your Google profile.
Google's blog announced that Social Search will be used to enhance Google results and will become a ranking signal. Social Search borrowed Hotpot's interface that annotates results with messages like "Dan rated this place 5 stars", so you can see why a page ranks so high.