Communities, like full-fledged Yammer, is meant for users who need to broadcast information to a wider group. The Communities app will allow users to share polls or questions at scale; notify employees of company announcements; and to see the reach and impact of those broadcasts. The Communities app also will allow broadcasts of town-hall type meetings with video, interactive questions and Q&A.
Users can get the Communities app from the app store that is built into Teams. It admins can opt to pin it for all users; manage who can install it via custom policies; or enable individual users to install and pin it themselves.
For those surprised Yammer is still around, yes, yes it is.