The Announcement bar is an optional feature that the Little Free Library volunteer steward can choose to enable. (The steward is the person responsible for maintaining the library.) It appears as a purple bar at the top of the Library Detail Screen.
The Announcement is a space for a short, timely message from the steward. It expires after 72 hours. For example, a steward may announce that they stocked up on bookmarks or added new books by a particular author.