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.