Selecting the Bookmark icon adds the course to your list of Saved courses. Your Saved courses can be accessed from the user menu in the navigation bar i.e found under your image dropdown in the top right corner.
You can Bookmark any course including finished, not currently available, and archived courses. Sometimes courses reappear (often updated) after months or years of absence. Some courses are eventually discontinued.
Class Central is a search engine for online courses and is not involved with running or scheduling courses. Bookmarking a course does not enroll you in the course. You need to go to the course provider site to enroll. See How do I enroll / register for a course?