What does plato mean by the ';true lie';?

im reading Rouses translations on the great dialogues of Plato and im not understanding it.What does plato mean by the ';true lie';?
The ';true lie'; is actually a bad translation of the Greek. ';Noble lie'; or ';grand lie'; works better (the Greek word is ';gennaios';, which is usually not translated as ';true';). It refers to a lie that the State tells the people in order to maintain social harmony. A good example is religion. If a State can get all of its citizens to agree on religion then that creates a sense of harmony that would otherwise not be there.





So, ironically, Plato endorses lying to the public if it means social harmony! What does this mean for his Cave Analogy? That in a sense it is good for us to ';be in the dark'; with respect to certain things? An interesting question...