3D Secure is a secured Internet payment protocol deployed by all major card schemes (“Verified by Visa” and “MasterCard SecureCode”) which limits fraud related to the fraudulent use of payment cards over the Internet. It is aimed at certifying that the card is actually used by its true holder.
When both the acquiring bank (Lemon Way bank in your case) and the cardholder bank are equipped, an additional step has to be performed by the cardholder when paying.
In addition to his/her card number, expiry date, and CVV number, the payer might have to enter a password, his/her birth date, or a One-Time Password sent on his/her mobile phone (depending on the cardholder bank choice/offer).
Even though 3D Secure has a proven negative impact on the payment conversion tunnel (addition of an extra step), the potential loss is generally largely compensated by the radical decrease in card fraud/chargebacks (that is normally supported by you).
A 3D Secure transaction does indeed lead to a liability shift, from the acquiring bank (the bank chosen by Lemon Way to process the transaction) to the cardholder bank. The transaction becomes non-repudiable (the cardholder cannot contest he/she was the one paying), leading to a reduction in possible card chargeback cases (product not delivered, etc.).
For more insights on 3D Secure, just take a look at the dedicated Lemon Way blog entry, here