The judgment of the jury is the final verdict. This verdict is an evaluation of the evidence as interpreted by the individuals who hear it. However, the individual biases of the jurors may easily interfere with justice. "The Monkey Trial" presented in the liteary work "Inherit the Wind" certainly portrays the affect that people's biases can have upon the judgment of people as many people look through the tinted glasses of convention rather than through the open window of conscience. (In the trial of a teacher who presented the theory of evolution, the fundamentalist/religious beliefs of the jury initially biased their judgment of the defendant.) Atticus hopes that the jurors will hear the evidence with their consciencs in "To Kill a Mockingbird."
We need only follow the news of high-profile cases to understand how important the objectivity and ethics of a jury are.
Atticus is implying that the validity of the trial system in this country is determined by the juries involved. Each person brings with them to jury duty a life time of experience that leads to a personal philosphy and agenda. These agendas are carried into the courtroom and can easily influence how the jury hears the case. One has to be concerned with what the jury is hearing and seeing in their own eyes.
Atticus' statement is one that suggests that the system will only work if the jury is honest and open to truly hearing the evidence.f He is suggesting that the jury must drop their own agendas, as difficult as that be and look with only reason at the evidence in order to make the best decision.
A further implication lies in the the idea that the court is the great leveler. The jury, defendent, spectator and lawyer are all equal in the situation. That equality is only guaranteed when those involved in the case respond openly and honestly without prejudice.