Community Messages is a feature where you can write several things to the community of the wiki. You can edit it on this page on your wiki: "MediaWiki:Community-corner".
You could do the following things:
- Organize a project to build up an area of the wiki
- Show off some new pages that everyone should see
- Get people involved in a discussion that's happening on the site
- Give a shout-out to a contributor who's added something special to the wiki
- Just post some words of encouragement