What does Atticus mean when he says that "you are the products of several generations' gentle breeding"?Harper Lee's To Kill a Mockingbird

In To Kill a Mockingbird, after Aunt Alexandra arrives at the Finch home, she is disturbed with the way that Scout is being brought up.  So, she prompts Atticus to remind Scout who she is and that their family is a prominent social family in Maycomb.  As such, Aunt Alexandra believes that Scout must conform to the ideas of what constitutes a young lady in higher Maycomb society.  Atticus talks with Scout about her "gentle breeding" as, ironically and humorously, she scratches a chigger which has gone up her leg:

"...Your aunt has asked me to talk to you... about the family and what it's meant to Maycomb Country through the years, so you'll have some idea of who you are; so you might be moved to behave accordingly," he concluded at a gallop.

As this manner of speking is uncharacteristic of Atticus, Scout is upset that Aunt Alexandra has coerced Atticus  into talking to her in this manner.  When she picks at a comb from her drawer and runs its teeth along the edge, Atticus snaps at her.  Scout turns, but runs into the vest of her father, where she hears his stomach growling (he is upset).  Solicitously, she urges him to "take some soda."  Then, she asks,

"Atticus, is all this behavin' an'stuff gonna make things different?...."

Her father tells her it is not time yet to worry.  So, Scout asks if she is really supposed to remember all that Aunt Alexandra told him to say. Then, Atticus tells her no:  "Forget it."

The chapter ends with the older Scout as narrator telling the reader that she now understands Atticus's motives, but such explanations are best left to a woman.  Aunt Alexandra is upset with Scout's wearing of overalls and her playing with the boys and not being "ladylike" since they are from a rather prestigious family, one that others watch and pass judgment upon, but Atticus feels Scout is still too young to be confined in dresses all the time.

This little episode exemplfies the genuineness of Atticus in contrast to his sister's rather pretentious attitude.  Clearly, Aunt Alexandra demonstrates a societal prejudice, holding tightly to what is considered traditional.