Whats it mean when white liquid come out after nipple piercing?

The white fluid is just your body healing, and nothing to be concerned about. You do want to clean it often so it does not get infected. Your biggest concern is when the liquid is green or yellow. At that point, consult a piercer for suggestions, most of the time, they will tell you to either use triple anti-bacterial ointment, or have you remove the jewelry and see a doctor about antibiotics.