It's a diary that keeps track of your food and exercise, and converts it all to calories! You will want to first figure out the number of total calories you should be eating in a day, depending on your height, weight, and activity level. Then, all you have to do is type in what you eat, or scan the barcode, (how cool!!) how much you exercise, and it calculates the number of calories and subtracts it from your daily total. You will always be able to see exactly how many calories you have left so you don't exceed your given amount.
But don't worry about all that, it's super simple to navigate once you download it to your phone. Then you can take it with you, so if you eat on the run, you can still keep track.