At Admin Panel -> Settings -> Switches, the "member personal guestbooks" switch enables a simple guestbook in the profile of each member. It's basically a thread of posts ordered with the most recent on top. Members can post to their own guestbook as well as those of others. Some forum software calls this "visitor messages".

Each member has an option on their Options -> Edit Profile page to choose whether they want to be notified of new posts to their guestbook.

If you're looking for a feedback mechanism, you should probably use the auction-style feedback switch instead. Guestbooks are just a little social networking touch allowing members to say things to each other without having to send a private message.