The favorites section appears at the top of the left sidebar. You can add conversations to this list that you communicate in frequently.
To add a conversation to your favorites list, click the gear icon at the top right of the room and select "Favorite".
If you haven’t favorited any conversations yet, the favorites section in the left sidebar will be automatically hidden. To unfavorite a conversation, click the gear icon at the top right of the room and select "Unfavorite".