What is the significance of Calpurnia in the novel "To Kill a Mockingbird"?

First, Cal fills the role of a mother figure for Jem and Scout.  While Scout and Cal may clash at times, Cal is deeply protective of Scout and loves her dearly.  

Cal also reinforces one of the main themes of the novel - to crawl inside someone else's skin before you judge them.  This is evident early in the novel when Scout has Walter Cunningham Jr. over for lunch.  He pours syrup all over his lunch.  Scout is shocked and makes Walter feel foolish.  Cal calls her into the kitchen and tells her that she has no right to shame a guest.  Here Scout begins to see that her view of the world is not the only one, for Walter never has syrup to put on any of his meals, so when he is offered the chance, he takes it.

Cal also serves to represent another key theme - race.  This is evident when Cal takes Jem and Scout to her church.  Here Scout learns more of the background concerning the Tom Robinson trial.  She also sees how the African American community has come together to support Tom and his family.  Here too Scout sees that Cal resides in two different worlds (much like Scout herself, for she is straddling the worlds of youth and adulthood and childhood and womanhood).  In the 'white' world, Cal is their educated mother-figure.  In the 'black' world, Cal is a mother to her own children and a member of the church.  This latter world is one Jem and Scout really never knew existed.