What is this, a homework question? Read the book and find out.
At the risk of outing myself, here is something to get you started:
Socrates is drawing an analogy between the just state and the just individual. In the just state, as in the just individual, each of the 3 major components is doing what it is supposed to do and not usurping the functions of the others.
But this is only a preliminary answer, for without “escaping from the cave” and grasping the Form of Good, we can never truly grasp the essence of justice. So the explanation of justice that Socrates offers in this work is tentative.
@Adirondackwannabe Socrates was Greek, not Roman.