It allows you to input your current weight, how much you want to lose, then calculates how many calories you can eat per day. If you do any cardio, you can add that, and it allows you to eat a little more.
The best thing about this is that it makes me stop and think 'do I really want to eat that'! At the end of the day, I click complete, and it tells me if I keep going, I'll weigh xxx in 4 weeks. A definite motivator.