What does enrollment in a MOOC mean?

In 2012, when MOOCs became largely known, media reports were fascinated with the big number of enrollments. The number 150,000 students was mentioned for both Stanford's Artificial Intelligence course and MIT's Circuits and Electronics, to be later followed by the underwhelming completion rates, that often are in the single digit percentages. But what kind of enrollment do these large numbers really show? We try to answer this question by breaking this number into its components, while comparing two successive iterations of the same MOOC offered on the edX platform.