If you especially like or a certain comment, you can applaud to it, which grants an extra dose of Fairy Dust to whoever wrote it.
Applauding to a comment can mean that you really agree with the message of a comment, that you find it to be especially funny or well written, or that you are grateful for this comment for any other imaginable reason.
Opposed to many other things on Belletristica, applauding is not anonymous. Your username will be shown in the list of people who have applauded to a comment.
By the way, you can not only applaud to comments for books, but basically wherever comments are possible. For example, you can applaud for entries on the pinboard on someone's profile.