If a course is set to offline, nobody except the course admins can access it. This means previously enrolled students can no longer access the course.

If a class is closed (while the course is still online), previously enrolled students can still access and complete the course, but no new students can enrol.

To set a course offline:

Step 1

Go to Course Setup > General

Step 2

Toggle from Offline to Online

To close a class:  

Step 1

Go to Course Setup > General

Step 2

Scroll down to Classes in Your Course and select Change Class for the open class you wish to close

Step 3

Select No one (close this class) and then Save Changes