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:
http://motionmailapp.com/pricing

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.