This convenient button on all Class Central course information pages allows you to keep track of all your courses in one place. You will find all your selected courses at https://www.classcentral.com/user/courses.
Selecting Interested adds the course to your personal Interested list. You will receive an email from Class Central when courses in your Interested list are about to open. You can select Interested for any course including finished, not currently available, and archived courses. Sometimes courses reappear (often updated) after months or years of absence.
Class Central is a search engine for online courses and is not involved with running or scheduling courses.
The Taking right now and Completed/Audited check boxes also add courses to your personal Class Central course lists. They do not enroll you in the course with its course provider. Enrollment instructions are here.
You can also create custom lists which you can make public, unlisted, or private. Public means that anyone can view the list and it may appear in search engine searches. Unlisted means that the list will not appear in searches, but anyone who has the URL can view it. Private means that only you can view the list.