"Community Bulletin" is the new block that replaces the "Visit Meta" block that was before used on Stack Exchange sites.
The Community Bulletin box is a tool for the community to promote events and initiatives that are important to them. On-site activities like tag cleanup events, chat events, and weekly topic contests are some examples of things to promote in the community bulletin.
Moderators and select Stack Exchange employees can add new events from the Mod Links menu, giving each entry a title and beginning and ending date & time. Each entry must be linked to a Stack Exchange property (a Stack Exchange main site or meta post, Blog Overflow blog post, or a post on the main Stack Exchange blog).
More information can be found here: