A body fat monitor is a scientific device that measures the percentage of body fat a person has. Many people think that being a normal weight means having a healthy amount of body fat. Wrong. I realized this when I got a body fat monitor for use in my work. I found quite a few slim people who had a high percentage of body fat. This isn't good for the health.
A small study carried out recently seems to show that people who are a normal weight but have high body fat (referred to as "normal weight - obese" in the study) may have an increased risk of heart disease. More information on this research is here.