Each credit gives you one address in your list to download.

For example: if you have a Metro Area plan, you get 2,000 credits per month.  That means you can download  up to 2,000 addresses each month.  If you download a list of 500 addresses, you will be able to download another list of up to 1,500 more addresses before your renewal date.  After your monthly renewal date, the number will reset to 2,000.