Depending on how much weight you are trying to lose(you set it up by picking easy to harder), you can be under budget, just right, or over budget at this point in time. This is calculated in real time, so as more time passed you will accumulate more calories and this might be under, over or just right. It spreads out your calories over the day. If you look at the front icon, it tells you how many calories you've eaten and how many you have left for the rest of the day...don't go over that number and you will end up under or just right for the entire day.
I was confused by this at first also, now I find it really handy 🙂
Here it is explained probably better than I can do it 🙂
Help article; How do I track my food with Fitbit
