I assume that you are talking about what happens in Chapter 14. There, Atticus tells Aunt Alexandra a number of things.
First, he says that Calpurnia has done a good job raising the kids. He says
If anything, she's been harder on them in some ways than a mother would have been... she's never let them get away with anything, she's never indulged them the way most colored nurses do.
He also tells Alexandra that the children love Calpurnia for how she has raised them.
He tells Aunt Alexandra that he will never throw Calpurnia out because she is essentially a member of the family
"Alexandra, Calpurnia's not leaving this house until she wants to. You may think otherwise, but I couldn't have got along without her all these years. She's a faithful member of this family...
By saying these things, he is clearly defending Calpurnia against Aunt Alexandra, who has come in part because she does not approve of a black woman raising Scout as she gets older.
Atticus does defend Calpurnia in Chapter 14 as previous answers conclude. Aunt Alexandra is worried that the children's association with Calpurnia is getting too casual because she invited them to her home. Scout would love to go see Calpurnia's home and pay her a visit, but Aunt Alexandra objects because she believes that her family should not associate with people outside of their social class. Scout overhears Atticus tell his sister that he will never get rid of Calpurnia for a few reasons: first, Calpurnia is not a threat and she's practically family; second, "the children love her"; and third, she's the closest thing to a mother that the kids know and she's taught them many good things (137).
Atticus also tells Alexandra that she should not have to work herself so hard anyway. Calpurnia does the cooking and cleaning, which gives Aunt Alexandra time to focus on the children as Atticus becomes very busy with the Tom Robinson case. Because Atticus is polite, yet direct with the way he phrases his conversation, he is able to lay down the law with respect. Aunt Alexandra still doesn't like his answer, though, and she takes out her frustration on her embroidery.
To Kill A Mockingbird
In one scene, Scout reports that she went to church with Calpurnia. Alexandra does not approve. When Scout asks Atticus if she could go to Calpurnia's house, Alexandra interjects and says "no." Scout, then, makes it clear that she did not ask her but Atticus.
Atticus, then, reprimands Scout by saying that she has to listen to Calpurnia and Alexandra.
In private, Atticus addresses Alexandra's complaints. He makes three points. First, Calpurnia is part of the family, as much as she is. Calpurnia has been vital to their family all of these years. Second, Calpurnia is a good woman and has done a great job in being a mother figure for Scout and Jem. Third, they need Calpurnia, especially as times are tough.
Atticus’s voice was even: “Alexandra, Calpurnia’s not leaving this house until she wants to. You may think otherwise, but I couldn’t have got along without her all these years. She’s a faithful member of this family and you’ll simply have to accept things the way they are. Besides, sister, I don’t want you working your head off for us—you’ve no reason to do that. We still need Cal as much as we ever did.”