Credits are the equivalents of cents US. For instance a 10 dollar payment would earn 1000 credits.
Credits can be purchased via single payments (for instance; a single payments of 7 dollar will give you 700 credits) or via monthly subscriptions. The latter are available on the subscription page. In addition, these monthly subscriptions yield 10% additional credits (e.g., a monthly subscription of 10 dollar corresponds with 1000 credits + 100 bonus credits = 1100 credits).Note that the single payment site only works after entering your credit card information in the settings page.
Credits that are not used (for instance credits from a monthly subscription that are not used that month) are kept and can be used for future analyses.