Scout, who is very young when the novel opens, is innocent because she has not yet internalized the values of the adult world. Her innocence is on open display in an early comic interlude when she inadvertently offends her new, out-of-town schoolteacher by already knowing how to read. The teacher condemns this because she fears that Scout will be polluted by having been taught the "wrong" method, highlighting the adult community's anxiety about controlling and molding youthful experience according to its own values. (Atticus teaches her both to hold to her values and outwardly conform.)
Scout is also largely innocent about the importance of gender norms, preferring to be a tomboy and spend her time with Jem and Dill rather than act like a lady.
Above all, however, Scout's innocence exposes the racism that infects the Maycomb community. Because she is too young to know better, she can see that Tom Robinson is being framed and even hope, though Atticus indicates otherwise, that Robinson could be found innocent. She respects Calpurnia as a mother figure with authority although Calpurnia is black, evaluating her on the basis of her character rather than the color of her skin. She is innocent, too, in not realizing until late in the novel that Calpurnia has a life in the black community outside her job in the Finch household.
Because she is a young child, able to see the world with fresh eyes and a child's innate sense of justice, she becomes an ideal vehicle for exposing the racism at the heart of Maycomb's social order.