Dear sangwoo Kim,
You linked your debit or credit card to your PayPal account on Dec 17, 2014. To make sure the card is yours, we made a small charge to it that you'll need to confirm (and we'll refund the money to your PayPal account when you're done).
The charge creates a unique 4-digit code on your card statement. If you don't see the charge right away, don't worry - sometimes it takes a few days to show up. If you can't check your card statement online, check your paper statement when it comes in the mail (this can take up to 30 days). You will find the charge from PayPal with a 4-digit code.