Moderation is the acceptance of new comments on lessons and homework assignments: the teacher marks the comment or the entire thread as moderated.
To moderate comments, open a course and go to the "Comments" tab in the bottom navigation. The list will show threads with new comments. Click the "mark as moderated" icon on the comment or thread to remove it from the moderation queue.
You can enable comment feeds in the course settings, in the "Comments" section.