This will let users get updates for certain actions in the News Feed, without having to subscribe or be friends with the user.
For example, users could choose to follow and get notified when another user posts a photo to Instagram or when a movie critic publishes a new movie review -- without having to see all of the various updates from that person. It's like subscribing to someone's news feed, except you get to pick what parts you see, based on what app you are using.
On its developer blog, Facebook's Yaran Sadan describes the process. There is also a documentation page for the new feature, which is called Open Graph Follows.
We can already see how this could be useful for some of the most popular Open Graph apps such as SocialCam and Viddy. I might not want to follow every update of a particular SocialCam star, but I might like to know when he or she uploads a new video.
It will be interesting to see how app developers integrate the technology with their existing networks. On some services with Open Graph implementation -- such as Instagram and Pinterest -- I follow users that are not my Facebook friends. We're interested in seeing how easy it will be to bring notifications from those users into the News Feed.