Admins can pin an important message in a community — a meeting time, a welcome note, or anything worth keeping easy to find above the regular flow of chat.
Pinning is one of the tools available to a community's owners, admins and moderators as part of running it well. See Roles in a community for the rest of what admins can do.
If you're a member rather than an admin, you'll be able to see pinned messages, but pinning and unpinning is managed by the community's leaders.
