Put simply Samsung Knox is a program that runs on newer models of Samsung Android devices (like the Galaxy S3 and S4) which provides a secure environment that is separate from the less 
secure Android operating system.
When you start KNOX you will see a new desktop, secure email, secured internal apps (possibly with Single Sign On SSO, if you buy Centrify’s add-on product) and secure file storage.
This video does an excellent job of explaining the basics:
