Meta courses are courses which take their enrolments from courses i.e. for every course 'enrolled' on the metacourse, all students in the course are enrolled in the metacourse.
Each time a student enrols on (or unenrols from) a course, s/he is enrolled/unenrolled from the metacourse(s) (although it doesn't happen immediately - it may take an hour to show up).
Teachers are able to choose whether a course is a meta course via the "Is this a meta course?" (yes/no) checkbox in the course settings. If a meta course is chosen, the students page changes from listing/searching for students, to listing/searching for courses.
The original discussion Meta courses experimental feature contains further explanation.