When you enter your card number, our payment systems check the card to make sure it is valid by submitting an authorization transaction on the card. This is only an authorization, so no money is actually deducted from your account.
For some banks and credit card providers, we're able to submit the authorization transaction for $0.00, but for Discover, American Express, and some MasterCard-issuing banks we have to submit another amount, usually $1.00. Because this is only an authorization, the charge is voided as soon as it goes through and no money is actually deducted from your account.
Banks vary at how quickly they remove these voided authorization requests, but if you do see a transaction on your statement for this amount it should be removed within a few business days.