Move ring

Do you mean starting a workout?;

1. Open the Workout app. 
2. Find the workout that best matches what you're doing. Learn more about each workout type. 
3. To set a goal, tap the More button  next to the workout that you want to do. To skip setting a goal, tap the workout.

Wait for the three-second countdown. To skip the countdown, tap the screen.

Because the different workout types have different algorithms depending on how strenuous the workout type is and because you didn't start a workout the default exercise algorithm was used instead.

Question7152 wrote:

I forgot to set my exercise

What does this mean as there is nothing to set?.