The editor tools module adds a toolbar to the Phorum message editor, which can be used by visitors to easily add things like BBcode tags and smileys to their postings. This is particularily useful for users unfamiliar with BBCode syntax. It is enabled by default.
Simply click on the button on the toolbar to insert the tags for the effect that you desire. By clicking on "Settings" in the admin panel Modules section, you can toggle whether or not for an additional help button to be displayed on the toolbar, which gives information on BBCode syntax and smilies.