[It] is the network of connections Google uses to identify relevant social search results. It is based on a combination of the following:
• Direct connections from your Google chat buddies and contacts
• Direct connections from links that appear on your Google profile
• Secondary connections that are publicly associated with your direct connections
In addition to web pages from your social circle, posts from your Google Reader subscriptions may also appear in your social search results.
(via Ethan Zuckerman)