Sales funnel
A sales funnel is the path from someone discovering your course to buying it. Each stage loses people, which is why it is drawn as a funnel: many enter, few purchase.
The usual stages are attention, interest, consideration and purchase. Measuring them separately shows where the sale is lost.
The most common mistake is confusing a traffic problem with a conversion problem. If a thousand people see the page and nobody buys, the answer is not more visitors: it is the page, the price or the offer.