3-D Secure is a way to verify online transactions and thus help to reduce fraud. 3-D Secure verifies whether you are the real cardholder.
After you have entered your card details, a new screen will appear in which further processing with your bank takes place. Here you will be asked to enter your personal security code. The relevant financial institution will verify the transaction within seconds and confirm that you are the person making the purchase.
There are different forms of approval with 3-D Secure. Depending on your bank (and country of origin) it can be used with a card reader or a digipas in combination with a pin code or a piece of information that only the cardholder can know.