With the bookmark module, you can allow users to bookmark topics and replies. Topics/replies that have been bookmarked by a user will appear on their profile under the bookmarks tab.
Users can remove bookmarks from the individual topic/reply or from their profile. The benefit of enabling users to bookmark topics and replies is that they can easily save and look at topics and replies that they find useful and important on their profiles.
Bookmarks is a basic module and is available via the basic, plus and pro plans. Modules cannot be bought individually.