If someone was to get hold of your card the last thing you would want is for them to have the opportunity to actually use it. 3D Secure is one last password check to make sure the person placing the order is the cardholder.
If you do not remember your password please do click the 'Forgot password' link in 3DS generated check box. This will then present a request for alternative verification like; full name, date of birth, and maybe another question depending on the bank. You will be given the opportunity to submit another password