He means that the justice system works only when each individual juror takes his duty seriously. This means that a juror must use reason and intellect, weigh the evidence objectively, and leave all biases and/or prejudices out of the courtroom.
Atticus's closing speech is found at the end of chapter 20 where he begins by saying that there aren't many complicated facts in this case. What the jury must remember, and what Atticus does not take time to retell for them is first, Mayella was beaten on her right side which signifies that the abuser was left-handed. Mr. Ewell is left-handed and Tom Robinson's left hand is crippled and unable to have hurt anyone. Next, there is no medical evidence showing that Mayella was raped by anyone, let alone by Tom. Finally, there are no secondary witnesses to testify for either side to say who is telling the truth. Therefore, Atticus shifts all of the blame to Mayella Ewell and places the word guilty upon her during his closing arguments. Atticus also blames the South's social and racial boundaries for the situation that the Ewells and Tom Robinson find themselves in that day.
