Levl is a device in which you blow for a short period of time in order to find out how much fat your body is burning during everyday activities. After a few seconds of breathing into it, Levl measures the level of acetone from your breath (the human body produces small quantities of acetone through metabolic processes), and then it displays it on the screen.
If you want to find out more about the calories you burned and the pounds you lost, you need the companion app. So, whenever you eat carbs, they transform into glucose and then into energy. Without carbs, the body starts to burn chemicals from the fats, which produces the acetone that is eliminated through breath.
Source: the verge