It's the only part not explained in the tutorial (I'm sorry!). The numbers are the turns, and the circles underneath them are the different actions in each turn. That way it's easier to tell which card/turn is the one that's being currently performed, and which turn(s) comes next.

It's also a way to know which turn/action is being performed right now, how many have already been performed(white), and the number of mistakes(red)/successes(green) you've made

I really should've explained it better in the tutorial. Thank you for playing!