I don't quite understand this Plato quote?

From the Republic, Plato wrote "Until philosophers are kings or princes of this world have spirit and power of philosophy, and political greatness and wisdom meet in one, and those commoner natures who pursue either to the exclusion of the other are compelled to stand aside, cities will never have rest from their evils, no, nor the human race." Could you please explain this? I don't quite get what he means.