Thank about this way. Our metabolism runs like a car that was just left running. If you just left the car running all day, it would use up some gas. Although, it would use up more gas if you drove it. Yes, this is exercise.
Exercise can mean a number of things.
It doesn't only mean an aerobic class or a treadmill. It means whatever you enjoy doing. It could be an aerobic class, gardening, sports, treadmill, stairmaster, walking, jogging, cleaning your house, swimming, taking the stairs, etc. All physical activities require energy just like driving a car requires gas. Back to our car analogy.