Calpurnia has raised Scout and taught her many things (how to write in cursive, for example). As her caregiver, Calpurnia is loving and yet strict with Scout. When Scout goes to Calpurnia's church, Scout is amazed at the way Calpurnia talks - shedding her "white" way of speaking. Calpurnia explains that she is being respectful and sensitive towards the other people in her congregation.
When Scout asks if she can visit Calpurnia at her house, Cal agrees.
Scout begins to understand that Calpurnia has a life outside of the Finch household. This realization is part of the maturation process for Scout (an all people, really).
Calpurina helped raise Scout and Jem, and at first Scout sees her only as an extension of herself, which the novel allows for by having Calpurina scold Scout in the beginning. Scout can't imagine Calpurina outside of her household, as being anything other than her Nanny. As Scout slowly begins to grow she realizes that Calpurina is her own person, that she has a life outside of her and Jem. This really comes to a head when Calpurina takes them to her Church. At the Church Calpurina takes on a persona that Scout has never seen, she uses a dialect and voice that is unfamiliar to Scout. Calpurina becomes the link between the black and white world for the children.