Most plans come with a default number of responses per month - credits let you go above this limit.
What Are Credits?
Credits are how usage is measured: one survey response equals one credit. Most plans included a default number of response credits per month, for example 200 per month. Sometimes a study can exceed the plan limit on a short-term basis, and it is better to add credits rather than upgrade to a larger tier.
How Can I Add Credits?
Credits can be easily purchased inside of Settings > Billing.