Pinned discussions allow instructors to save discussions s/he would like students to see first. If there is a discussion post students should be focused on for a particular week or month, the instructor or TA can pin the discussion so that all following discussions will appear beneath that of the one that is pinned.
By default, discussions are organized based on most recent activity. For pinned discussions, instructors can organize the threads in order of importance.
For more information on how to pin a discussion, please see the guide here: https://community.canvaslms.com/docs/DOC-2717