This quotation occurs in Chapter 6 of Harper Lee's novel To Kill a Mockingbird. The children, Jem and Scout, have managed to sneak into the grounds of the Radley place and are exploring. Nathan Radley, Boo's older brother, hears intruders but does not know their identity. He assumes that the intruders are black people trying to steal collard greens from his vegetable garden. He fires a shotgun blindly to scare off the intruders and the children run away.
Later, after Nathan recounts the story to a crowd that gathered after hearing the noise of the shot, Miss Stephanie comments:
"Scared him pale. Says if anybody sees a white nigger around, that's the one."
Both Nathan's mistaken assumption about the identity of the intruders and Miss Stephanie's comment are intended to convey to the reader the racial tensions and racism present in the white community.