The number of your joy points is displayed on the right site of the word "Rewards". One joy point is equal to one calorie.

You receive the joy points for each activity or a kilogram that you lost. Collect the points and choose a snack for yourself. The progress bar shows you how many joy points you have already collected and how many are still missing. When you have reached your goal, click on "Enjoy" button. The corresponding calories and the food will be added to your daily budget then.
For the best results, use them once a week, for example when you are with your friends, so that the body does not go into energy saving mode.