What does Mrs. Flowers tell Marguerite in the first part of her "lessons of living"?

Mrs. Flowers introduces Marguerite to a number of new ideas during their first meeting. On the walk over to her house, Mrs. Flowers tells Marguerite first that

Language is man's way of communicating with his fellow man and it is language alone which separates him from the lower animals.

Mrs. Flowers also tells her that

Words mean more than what is set down on paper. It takes the human voice to infuse them with the shades of deeper meaning.

When they arrive, she gives Marguerite the first of what she calls "Lessons of Living."

She said that I must always be intolerant of ignorance but understanding of illiteracy. That some people, unable to go to school, were more educated and even more intelligent than college professors.

This lesson is particularly poignant because earlier in the chapter, Marguerite berates and acts ashamed of her illiterate mother for using the wrong verb. As Mrs. Flowers continues to say, the poor have some very wise sayings, which Marguerite would do well to listen to carefully.
Ironically, Marguerite really only listens carefully to Mrs. Flowers because she sees her as educated and refined—nothing like her mother. When Mrs. Flowers reads aloud from the book A Tale of Two Cities, Marguerite claims that she adores the sound of Mrs. Flower's voice but "hadn't really heard, heard to understand, a single word."
The meaning behind Mrs. Flowers' words are slightly lost at the end of the chapter when Marguerite's mother beats Marguerite for saying "by the way" because, her mother says, only "Jesus is the way." At that point it must seem to Marguerite that illiteracy is in fact ignorance, and perhaps Mrs. Flowers doesn't quite understand the lower classes as much as she thinks.

When Marguerite first visits Mrs. Flowers, she doesn't know what to expect. She is surprised to notice that Mrs. Flowers's house smells like food, since she never imagined her doing normal things like eating. She is amazed and overwhelmed by all the sights in her home, from pictures on the wall to white curtains in the windows.
Mrs. Flowers offers her cookies and tells her she must:

always be intolerant of ignorance but understanding of illiteracy. That some people, unable to go to school, were more educated and even more intelligent than college professors. She encouraged me to listen carefully to what country people called mother wit. That in those homely sayings was couched the collective wisdom of generations.

Then, Mrs. Flowers reads to Marguerite from A Tale of Two Cities. Marguerite is overwhelmed by the beauty of hearing the older woman read. Mrs. Flowers lends her a book to read and invites her back to recite the poems on her next visit. Marguerite leaves feeling "liked" and "respected" and as if someone finally sees her as special.