White Hat Hacking is another name for Ethical Hacking. Ethical hackers or white hat hackers hack into a system with prior permission to find out vulnerabilities so that they can be fixed before a person with malicious intent finds them. A white hat hacker should think like a malicious hacker and try to hack the system as a malicious hacker would. And that's why they have to have in-depth knowledge of Hacking techniques, network concepts, Operating system concepts, etc. If you are looking for guide of how to learn Ethical Hacking, then check this link.