Community messages is a feature that allows admins to highlight things that are important for the local community to know about. For example, this space can be used to organize a project, show off new pages, or highlight an ongoing discussion.
Step by step
The community message is located on the Wiki Activity page and can be edited by admins.
If you are an admin, you can change it by clicking on the "edit message" in the message. This will bring you to MediaWiki:Community-corner where you can edit the content and add as much information as you want. After publishing the edit, your changes will appear immediately.
Users who are active on the wikia up to 24 hours after the community corner is changed will receive a notification about the change, similar to the notification for message wall messages. These will disappear after 24 hours if not dismissed by the user.
Minor edits to community messages will not trigger notifications.
If you would like to change the notification message itself (to perhaps make it more tailored to the colloquialisms of your wikia's topic), you can edit MediaWiki:Communitymessages-notice-msg.
We do recommend admins be careful not to add too much to the messages, as simpler messages are often easier to understand.