Aunt Alexandra is thankful to Miss Maudie for basically shutting down Mrs. Merriweather. Scout by this point in the meeting is somewhat in a daze and not really sure what the women are talking about, so she doesn't pick up on how rude or critical Mrs. Merriweather is being, but Miss Maudie most certainly does and silences her with just a few words. For this, Aunt Alexandra is extremely grateful - and it also gives the reader a kinder, more sympatheric view of Aunt Alexandra because the reader finally sees how much Aunt Alexandra really does care about her brother.
Mrs. Merriweather starts by complaining about how "sulky" her maid Sophy has been since the trial and how upsetting this is to her - she wants her maid to be cheerful. She goes on to say to the women,
"I tell you there are some good but misguided people in this town. Good, but misguided. Folks in this town who think they're doing right, I mean. Now, far be it from me to say who, but some of 'em in this town thought they were doing the right thing a while back, but all they did was stir 'em up."
What Miss Maudie recognizes that Scout doesn't is that Mrs. Merriweather is talking about Atticus. She thinks he "stirred up" the black community by giving them hope - by making them believe they deserved a fair day in court - and now the Blacks in Maycomb are sulky and dissatisfied. While moments before she was praising J. Grimes Everett for his work in Africa to help the Mruna tribe rise up from poverty and suffering, she is here criticizing Atticus for trying to do the same thing back at home.
Miss Maudie shuts her down by saying, "His food doesn't stick going down, does it?" Scout is now brought back into the convesation. She sees that Miss Maudie is very angry, and "her gray eyes (are) as cold as her voice." After this remark, Mrs. Merriweather "redden(s), glance(s) at (Scout), and look(s) away."
In a few shorts words, Miss Maudie has reminded Mrs. Merriweather and the other women of the obvious: they are in Atticus's house, eating Atticus's food, and doing so in front of his daughter. How dare they criticize him here?
While Aunt Alexandra, as hostess, wasn't sure how to change the conversation, she finds now she doesn't have to - and she is very thankful to Miss Maudie for her help. She truly is worried about her brother and concerned for his family, and here the reader sees that truth - and also sees the great hypocrisy of Mrs. Merriweather.