This chapter gives us some quick background on Aunt Alexandra:
She has a son who looks like a horse, and she is married to a man—but they don't live together.
Alexandra has lived with Atticus ever since his oldest son, Jem, died.
Yes, for all you Mockingbird fans out there, Jem's dead. No, we don't know how yet.
After Jem died, the family's cook, Calpurnia, left and never came back.
Atticus sold their house and moved across town. An ice cream parlor now stands where their house used to be.
Aunt Alexandra is "giving a Coffee" (3.34) for Jean Louise, which means a bunch of annoying girls her age will come over (boo!) drink coffee (yay!) and gossip about husbands and babies. (Um, where's the door?)
She continues forcing herself into Jean Louise's life by saying that she will never be allowed to marry Henry.
Even though Jean Louise doesn't actually want to marry, she is mad that she is being told not to.
Alexandra's reason: Henry is the child "of rednecked white trash" (3.72).
She also thinks that Henry is trying to "take over" (3.75) Atticus's law firm.
Jean Louise is fed up. She has nothing to say, other than telling Alexandra to go pee in her hat.
That night, Atticus gently rebukes her before Henry shows up, and Jean Louise goes out on her date with her favorite piece of trash.