Who were the two white women that Zora met, and why were they at her school?

In Chapter 4, Hurston recalls that "two young ladies just popped in" one afternoon when she was at school. She says that white people would often bring their friends, "who came down from the North," to visit the village school, because "a Negro school was something strange to them." We, therefore, assume that these two white ladies are from the North, visiting friends in Florida, and curious to see "a Negro school." However, these particular ladies are different because they arrive unannounced.
Hurston says that the two ladies both "had shiny hair, mostly brownish" and that one of them was "dressed all over in black and white." However, she was most attracted by and curious about their fingers, which she describes as "long and thin, and very white." Hurston reads for the two ladies, and they are very impressed.
The ladies, Mrs. Johnstone and Miss Hurd, invite Hurston (or Zora, as I'm sure she would have been known to them), to the hotel they are staying at and give her "strange things, like stuffed dates and preserved ginger." The ladies then have their picture taken with Zora, and they give her one more present, a cylinder stuffed with "One hundred goldy-new pennies." The next day, more presents begin to arrive, including "an Episcopal hymn-book bound in white leather," "a copy of The Swiss Family Robinson," and, finally, "a huge box packed with clothes and books."
The two ladies return to Minnesota about a month later, and we hear no more about them. We can only assume that they were two ladies visiting friends in Florida, curious to look around "a Negro school," who became particularly fond of Zora after hearing her read.

The two women that Zora meets are from Minnesota. They turn up at her school one day, and as one of the best students in class, Zora is chosen to read to them. The ladies are so impressed that they invite Zora to have lunch with them at their hotel. After lunch, and after another reading test, this time from a magazine, they give Zora a gift of some books.
This is an important episode in Zora's life, as it kindles in her a life-long love of reading. Not only that, but it broadens her cultural and intellectual horizons, making her realize that there's a much bigger world out there beyond the cramped confines of the small town in which she lives.