Atticus disagrees with Aunt Alexandra over how to raise Scout.
Aunt Alexandra thinks Scout should be raised like a little lady. Atticus, on the other hand, lets her wear overalls and go by a nickname, rather than her real ladylike name.
Aunt Alexandra's vision of my deportment involved playing with small stoves, tea sets, and wearing the Add-A-Pearl necklace she gave me when I was born; furthermore, I should be a ray of sunshine in my father's lonely life. (ch 9)
Alexandra feels that Scout and Jem do not understand their heritage. They should appreciate that they are Finches, and they have an obligation to be upstanding members of society and protect their family name. Atticus has raised his children to appreciate character more than status.
Scout does not understand or get along with Aunt Alexandra, because she feels that her aunt does not take the time to get to know her or appreciate her.