The Medium Partner Program is a program that allows writers to earn money based on the "member activity" of their articles. For example, Medium members can interact with your story by reading, clapping, highlighting, or replying. All of these factors are taken into consideration when calculating the earnings for a particular article.
Their breakdown is as follows:
Engagement Points + Follower Bonus + Read ratio adjustment x Read-through rate adjustment = Earnings
More details can be found at the link here: