Every four hours you will receive a Time Bonus in the form of credits. As soon as the countdown has finished on the Game Selection page you can collect the bonus by clicking on the Bonus button (below in the middle). The bonus amount depends on your level - the more you play the more bonus you will receive. You will receive even higher bonuses if your Facebook friends join in.
You can set a reminder to avoid missing a bonus. The Reminder button can be found on the Time Bonus page. Alternatively you can enable reminders in Settings.