This feature allows you to request the Braintree payment gateway to challenge the 3D Secure enrolled card holder password. Usually the cards are validated based on their number, expiry date and its Card Secret Code (aka CVV, CVC, CSC, CID, etc). With 3D Secure layer the card issuer (eg. Visa, MasterCard, JCB or AmericanExpress) adds an authentication step for online payments.
Why is this feature important? From Braintree website:
In order to lower the payment risk of your web shop this plugin adds other 3DS related features: Allow only 3D Secure cards, Ignore 3D Secure if AVS and Risk limit for non 3DS cards.