The Apple Watch uses various sensors to monitor your movement and track your progress towards healthy goals that the Apple Watch or an Apple Watch app will help you set.
You can see this progress across three categories; movement, exercise and time spent standing. Movement measures calories burned, exercise is when you hit 30 minutes of exercise in a day and the stand section aims to make you less sedentary. The watch will alert you to stand up if you sit for too long and nudge you towards movement and exercise goals.
You can also see the fitness information on your iPhone in the Health app.