3-D Secure is an online transaction authentication method, which you can best compare with entering a PIN-code or writing a signature for a transaction on a physical terminal (e.g. in a shop or a restaurant).
3-D Secure allows you, as a merchant selling online, to demand your customer to identify himself as the rightful owner of his (credit) card. Ultimately, the goal is to reduce fraud cases.
There are several forms of 3-D Secure authentication. Depending on the customer's bank and originating country, this can be in the form of using a card reader or digipass, entering a PIN-code, or even entering a piece of data that only the cardholder can know.
For more information about 3-D Secure (Fraud detection documentation), go to the Back-Office under the support tab.