In Chapter 30, Atticus and Sheriff Tate discuss and analyze Bob Ewell's attack. Atticus believes that Jem was responsible for Bob Ewell's death and says that he will refuse to cover it up. However, Sheriff Tate disagrees with Atticus and knows that Boo Radley stabbed Bob Ewell. Sheriff Tate implies that Boo was responsible for Bob's death, and he gives his reasoning as to why he will tell the community that Bob fell on his own knife. He explains to Atticus that the entire community would thank Boo for saving the children by knocking on his door and leaving gifts at his doorstep. Sheriff Tate considers the unnecessary attention that Boo would receive to be a sin, given the fact that Boo is such a shy individual. Dragging Boo into the limelight would essentially be harmful. When Atticus asks Scout if she understands Tate's reasoning, Scout says,

"Mr. Tate was right.... Well, it'd be sort of like shootin' a mockingbird, wouldn't it?" (Lee, 169).

This quote is significant because it depicts Scout's moral development and maturity. Throughout the novel, mockingbirds symbolize innocent beings like Boo Radley. Scout applies Atticus's earlier lesson when he said that it was a sin to kill a mockingbird. Scout understands that exposing Boo to the limelight would be similar to shooting a mockingbird because Boo is a symbolic mockingbird. 

Then Mr. Underwood's meaning became clear: Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men's hearts Atticus had no case. (Chapter 25)

In many ways, this quote sums up much of the meaning of the entire novel.  First, it reveals a huge moment of growth for the narrator Scout.  This quote shows that she finally understands what has been going on in her town in the way of prejudice and injustice all along.

This quote reveals the character of Mr. Underwood, the editor of the local paper.  In a scathing attack against the outcome of the trial, Mr. Underwood shows his respect for Atticus and his anger toward the biased system through which Tom Robinson was convicted.

Finally, this quote sums up the general attitude of a prejudiced south, both in the novel and historically speaking.  The metaphor of the "secret courts of men's hearts" is so pin-pointed to reveal that even in what America boasts of as a system of justice, there is a secret place inside of every human that simply cannot let go of prior experience, prejudice, or weakness toward or against something they believe in.  The facts alone had more than adequately proven Tom's innocence, yet a jury of his "peers" found him nevertheless guilty.  This moment, and this quote, show that Scout finally understands this.