I am assuming that you mean the aftermath of the trial of Tom, Chapter 21.
If so, we learn that Atticus' belief in the justice system is shakable. Despite proof that Tom could not have committed the rape of Mayellla because of his lame arm, the jury overlooks the evidence and votes Tom guilty. The only explanation is racism and it infects the town and their decisions.
As for Jem and Scout, they too become acutely aware of the deep injustice that has been handed down. For the first time, perhaps, they understand that even their good and just father cannot remedy all the wrongs in the world.
Here is a portion of this powerful scene, as the verdict is being read:
"I shut my eyes. Judge Taylor polled the jury: "Guilty...guilty...guilty...guilty..." I peeked at Jem: his hands were white from gripping the balcony rail, and his shoulders jerked as if each "guilty" was a separate stab between them."
As stunned as the children are, Calpurnia makes sure they understand that Atticus still deserves the utmost respect. As their father exits the courtroom, she commands: "Miss Jean Louise, stand up. Your father's passin'."