Secure folder comes inbuild with S8 , S8 , S9 , S9 , NOTE 8, NOTE 9. Secure Folder is a place where you can keep your Photos, Videos, files, apps,  etc  private and can be only accessible with your pin or biometric authentication. Its an encrypted space leveraging with defense-grade Samsung Knox Security platform, protecting your important information from malicious attacks. 
Secure Folder is only available on devices running Android Nougat 7.0.