Comments in a course are discussion threads on the lesson page and on the homework answer page. They are enabled in the course's learning settings.
To configure student comments, open the Courses section, select a course, and go to the "Comments" section of the Learning Process settings. Enable the desired checkboxes: show comments on the lesson page and/or on the assignment response page. Click "Apply."
Moderation of comments already left is done in the "Comments" tab of the course, not in this block.