3D secure is a card verification system used to increase the security of online transactions using credit and debit cards. It is the industry standard protocol for online CNP payments and transactions, mandated by the Card Associations. It is used to verify the person using the card as the owner of the card to protect against fraud and theft. Slide requires card holders to undergo the 3DS verification process when loading a card, however this is administered by an official third party service for obvious security reasons.
You can find out more about 3D secure by visiting https://www.vcs.co.za/profile/3dsecure.asp