Fitness trackers work by using accelerometers worn on your wrist, waistband or shoe to count your strides and convert that to an approximate measure of the distance travelled and calories burned.
Some also include altimeters, giving you extra credit for climbing stairs and hills
A 2014 study at Iowa State University compared them to more sophisticated laboratory equipment and found that most fitness trackers have error margins of between 10-20 %.
Over a period of a week, that error adds up to about one day’s worth of exercise.
Thank you for your useful information. I feel like science is getting better. :D