Announcements are important messages shown to all users and on all (or specified) pages which should stand out over the regular posts.
Since Phorum-5.2 announcements are handled as a module which is distributed with Phorum. See Section 3.2, “Phorum Modules” for generic module instructions. The module allows for better separation of posts and announcements and also some additional features like hiding announcements after a given time and it opens the way for other future announcements modules which offer even more functionalities.