What's the Noble Lie?

The noble lie is one of the most famous, i don't know, phrases in plato. I think people use it almost synonymously for a white lie, which is not at all what he's talking about. It seems to be part and parcel of what people take to be the worst aspects of plato's political philosophy. But there are two i suppose, we should say, but maybe we should situate it first. And that i dated a girl, actually, briefly in college, anda, she learned that i was interested in plato,. She got very upset with me one night, because how could you possibly be interested in somebody who defends such an abhorrent doctrine