For Exercise Timer to track your workout you must first create a custom workout. A workout is essentially a set of exercises which play one after the other to guide you through your workout.
Reps Exercises VS Timed Exercises
Exercises can be of 2 types:
1) Timed Exercises
Eg: 1 Minute Jumping Jacks
This is the most common Exercise. By just setting the exercise duration Exercise Timer will time your exercise and will let you know when the exercise is done.
2) Reps Mode Exercises
Eg: x20 Reps of Pushups
If your exercise is not time based, but is instead repetition based, you want to enable reps mode. During a workout Exercise Timer will flicker the screen when the current exercise is a reps based exercise. When you finish the exercise tap on the screen and you can move on to the next exercise.
If you want Exercise Timer to calculate the total workout time for you, it is important to still set the estimated duration of a Reps based exercise. Exercise Timer will use this to calculate the total workout time.
You can repeat all the exercises in your workout by setting the number of laps. If you set this value to 3, your exercises will repeat for 3 sets.
If instead you wish to repeat only a sub-set of your exercises you need to put them inside a group. A group is like another sub workout. It allows you to repeat a set of exercises for a number of laps.
Calorie Burn Calculation
Every exercise you perform burns calories. The number of calories burnt depends on the type of exercise, your weight, age, gender and more variables. For Exercise Timer to estimate the calories you burn in your workout you need to specify an MET value for each exercise. An MET value (Metabolic Equivalent Task) is an excellent universal measure of energy burnt during exercise.
You can read more about this here: https://blog.exercisetimer.net/2019/04/26/what-are-mets/