If you enable the Enable favorites web part property of the Forum group web part, forum posts will be displayed with the Add post to favorites link, as you can see in the screenshot below.
If a user clicks this link, the post will be added to their favorite posts.
If you place the Forum favorites web part to some page, the web part will display links to favorite posts of the current user. You can see the appearance of the web part with two favorite posts in the screenshot below.
Page url: http://devnet.kentico.com/docs/devguide/index.html?forum_favorites.htm