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.