The subject you asked about is worth writing a book, a thick one.

Simplifying, you earn money when I pay you for something that is of value, worth to me more than I pay you. I make a profit; you earn money.

Contract work is the simplest example. In employment, too often, there is a lack of clarity.

In the case of the Medium editors, it appears to me that Ev did not have any measurable targets. One needs to trace month by month in dollars the value that editors bring to Medium. Then, they can try a few things and avoid the surprises of massive buyouts.