Under Foursquare's process of checking in, users can tell followers where, physically, they are. The information can be syndicated to Facebook or Twitter. People who are checked in at the same location can engage in conversation.
According to a report in Techblog86, a China-based site that first broke the news of China's decision to block Foursquare, people who checked in to Tiananmen Square were leaving "sensitive comments" on the page for others to see. Those comments were shared with friends and followers on Facebook and Twitter. China, objecting to such comments, decided to block the service in an attempt to quell the protest.
China has yet to confirm that it has in fact blocked Foursquare. The social network did not immediately respond to request for comment.