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).