Every ListIQ plan comes with a set amount of monthly available credits. It uses credits to generate research info for journalists and publications. Each month your credit allotment will refresh to the max allowed by your plan.
You can review available credits in the extension itself to see both how many credits are being used to research information about a given list based on the research info you've selected as well as to review how many overall credits are available to your account.
You'll notice that ListIQ uses two types of credits: Email credits and Standard credits.
As you might suspect, Email credits are specifically used when finding email addresses, and standard credits are used for everything else.