"When Aunt Alexandra went to school, self-doubt could not be found in any textbook, so she knew not its meaning. She was never bored, and given the...

This quotation comes from Chapter 13 of To Kill A Mockingbird. Aunt Alexandra, Atticus's sister, has come to Maycomb to watch Jem and Scout during the long summer, and they discover that she is excessively proud of her heritage and ways of being and not open to other ways of thinking.

Unlike Jem, Scout, and Atticus, Aunt Alexandra "fitted into the world of Maycomb like a hand into a glove" (Chapter 13). She upholds all the local values, such as supporting missionary societies and hosting parties for the local ladies. Aunt Alexandra also likes to disparage other families and considers her family, the Finch family, to be superior to others. She is disappointed that Atticus has not taught Jem and Scout to appreciate the family and its heritage as she does.

Unlike Atticus, who questions what he sees around him and who judges people as individuals, Aunt Alexandra, as the quotation in the question implies, sees the world in a black-and-white fashion. She never doubts her judgments and her sense of what's right, and she considers other ways of being and other families inferior. Scout and Jem are not used to this type of rigid thinking, and they chafe against Aunt Alexandra's rules and dictates. Atticus is not so sure of himself, and this self-doubt makes him a more humane and open person than Aunt Alexandra.