What news does Atticus bring about Tom Robinson and what does he explain about what Tom has done?
In Chapter 24, Atticus interrupts Aunt Alexandra's missionary circle to ask Calpurnia if she could take him to see Helen Robinson. Atticus proceeds to tell Cal, Alexandra, Miss Maudie, and Scout that Tom has been shot dead. Atticus explains that during the inmates' exercise period, Tom charged towards the fence and began climbing it. After being instructed to stop, the prison guards fired a few warning shots before they aimed at Tom. The prison guards ended up shooting Tom Robinson seventeen times, killing him during his escape. The women are visibly shaken after hearing about Tom's tragic end, and Atticus tells them that he thinks Tom was sick of "white men's chances." Despite having the opportunity to appeal the ruling, Tom decided to take matters into his own hands by attempting to escape. Unfortunately, Tom Robinson was shot multiple times by the prison guards and died while he was attempting to escape.
Because Atticus respects Tom Robinson and his wife and family, he personally brings Helen the sad news that Tom has been shot trying to escape. Tom is shot 17 times by the guards at the prison as he was trying to climb (with one good arm) the surrounding fence. Atticus tells Helen that he feels Tom understood he didn’t have a chance in an appeal, so that is probably why he tried to escape. Tom’s escape attempt says a lot about Tom’s character. He did not want to leave his family destitute and unable to provide for themselves. Atticus takes Calpurnia with him when he delivers the news in hopes that Calpurnia can give Helen some comfort. As Atticus breaks the news, Jem and Dill witness the tender respect Atticus gives to Tom’s family.
I assume you are referring to Chapter 24, when Atticus comes to tell the family that Tom was shot while trying to escape from prison. Always trying to understand another's perspective, Atticus explains that Tom lacks any hope for an appeal after having been convicted by a jury that could not have believed he was really guilty.