Credits are used to keep track of your usage of MotionMail. A credit is used up every time someone views an email containing your countdown timer - an impression. The number of credits you can use up each month depends on the plan you choose. 

You can view the plans which are available here:

If you use more than the number of credits you have left in your account, an overage charge will apply. Some plans allow credits to roll over to the next month.