3D Secure is an extra layer of security in using your credit card for online transactions. For online transactions over R1000 you'll be asked to enter your 3D Secure password (different from the PIN), which is called Verified by Visa and MasterCard SecureCode by the respective credit card companies.
3D Secure is a feature you need to activate with your bank. You can read more and find out how to activate 3D Secure here: