Lula is a black woman whom the children see when they go with Calpurnia to the black church. She is described as follows:
... standing in the path behind us was a tall Negro woman. Her weight was on one leg; she rested her left elbow in the curve of her hip, pointing at us with upturned palm. She was bullet-headed with strange almond-shaped eyes, straight nose, and an Indian-bow mouth. She seemed seven feet high.
She appears as a rather forbidding figure in this description, what with her 'bullet-head' and her height; to the awed young Scout she looks all of 'seven feet high'. She also acts in an aggressive manner which would seem to be in keeping with her appearance. She demands to know why Calpurnia is bringing white children to the black church, declaring truculently that Calpurnia hasn't any right to do this. From this it would appear that she believes in a policy of strict racial segregation, just as so many whites do in Maycomb. Calpurnia points out reasonably that even although there might be different churches, 'it's the same God' for people on both sides of the racial divide.
Jem and Scout are intrigued when they hear Calpurnia talk to Lula in a different way from her usual speech. She uses a black dialect, 'coloured folks' talk' as she later describes it to the children. She talks in this way when she is in the black part of town.
It is her use of this new idiom, or manner of speech, that first makes Scout realize that Calpurnia has a whole different life outside of her life as the servant of the Finch family. In this sense, she leads a double life. She talks and acts differently when surrounded by other black people, from the way that she acts and talks in the white Finch household. Scout and Jem have never heard her talk 'coloured folks-talk' before because they have never been with her in the black part of town before.