Haper Lee's To Kill a Mockingbird is set during the Great Depression just after the stock market crash. Early in Chapter 2, Atticus explains to Jem that the stock market crash hit the "country folks," meaning the farmers, the hardest. As Atticus further explains, Mr. Cunningham is unable to make money from his farm because much of his land is entailed and the rest is mortgaged; plus, he would rather go hungry than leave his land to work some other job.
What we also know about the effect of the Great Depression on farmers is that food prices dropped drastically, resulting in very small profits. Prior to the Great Depression, food prices had risen along with need because fighting in Europe during World War I caused Europe to need to import foods, resulting in significant profits for US farmers. US farmers were even encouraged to increase food production, which required taking out loans to buy more equipment and acres of land. However, after WWI ended, food prices dropped significantly when Europe became able to produce its own food, leaving farmers with small profits and deeply in debt ("U.S. Farmers During the Great Depression").
Hence, Lee is referring to the financial devastation farmers experienced during the Great Depression when she refers to "country folks," like the Cunninghams, as being the poorest and suffering the most in To Kill a Mockingbird.