If a student posts a comment on one of your lessons you can then reply to their comment either privately or publicly. When you reply to their comment they will be notified of your reply and if you have public comments turned on then your reply will be featured with their comment on your lesson publicly.
Go to the 'Comments' menu bar option then navigate down the page until you find a new comment:
There will be a 'reply' link.
Click that and it will open a dialogue box. Write a reply within that dialogue box and then click on the button 'Send'.
You've now replied to the comment. Depending on your comment settings your reply will either be posted privately or publicly for your student/s to view.