Credits are used to create Prepaid Burners within the app. However, they can not be used to extend Prepaid Burners. If you wish to keep the number, you can upgrade to a subscription with your existing number.
Purchasing New Credits
The app will prompt you to purchase more credits if you do not have enough credits to create a new Burner.
Then choose how many credits you wish to buy. Credits are sold in "packs" of 3, 8, 10, 15, or 25.
Your app store may request that you login or confirm your password before you can purchase.
Once you have the credits, you can use them to create a new Burner.
Keep in mind that Prepaid Burners are always assigned an expiration date. If you want to keep the Burner, be sure to upgrade to a subscription.