Each and every day you play the game, you can log in and receive a reward - it might be Materials, XP points, Guano, or even Potbucks! If you make sure to log in and collect your reward every day, on the 7th day you get a WHOPPING 15 Potbucks!
And that's not even the best part: on the 8th day, you start all over and can collect your week of rewards again!
If you miss a day, your reward counter will start over from the beginning again. Don't worry though -- you'll always be able to start over and you'll always get a reward!! :D