Note that this is an org unit setting, which means that it applies to all discussion messages within that course offering.
Once turned on, each user can apply a 1 to 5 stars to each message. The average rating of each message appears on the list of messages, and the number of ratings for a particular message is displayed when the message contents are visible. It is not possible to see what rating a particular user has applied to a particular message. Users can change their ratings of messages at any time.