Archiving a course, will move the course and its assessments to an 'Archived Course' tab on the left-hand panel. All of your archived courses, assessments, and results will still be able to be available from the left-hand panel (and can be 'un-archived' at any point), but they will be removed from your 'Course Dashboard'.
Image: Archived Courses tab on left-hand panel
To archive a course....
Select the course from the left-hand panel,
Click the 'Course Actions' dropdown menu, and
Select 'Archive Course'.
GIF: Select Course > Course Actions > Archive Course > Archive