In my mind, claps and comments are what constitute engagement on Medium. Maybe other people think of it differently.

Ultimately, though, what matters in terms of eyeballs is what exactly the Medium algorithm considers a signal to boost a particular story. Unfortunately, only the engineers at Medium know the answer to that. Still, I am quite sure click through rate, read rate, number of claps, and number of comments all influence to how many additional readers a story is pushed out.