What information do we learn about conditions in Alabama in 1935? Why might this information be significant to the plot?
The information that we obtain which is directly important to the plot is the following:
The plot is based in the city of Maycomb, a place which like the story says is "tired with time" (stuck it its ways so to speak), and in which blacks and whites were separated. Furthermore, it was a very prejudicial town in that even the white families discriminated against each other just for the sake of their last names. Note how the "Ewells were considered white trash", the "Haverfords were considered jack asses" and the "Cunninghams were poor".
On top of the white on white prejudice there was the imposing white on black prejudice, since the blacks had no basic human rights.These were the days prior to MLK and Rosa Parks, and the black race was still treated as second class citizens, admittedly by many, and quite openly.
On another note, we even see prejudice in the form of Ms. Caroline, the teacher,who is repulsed by the young boy Ewell, and who herself is rejected by the rest of the town from being from North Alabama, which is a town of "peculiarities".
Concisely, the first part of the novel shows us a place in which nobody is safe from criticism, pressure, dislike, prejudice, hatred, stereotyping, and disdain. For this reason, Robinson's trial was all the more poginant, and all the more scandalous and powerful. It was like exploding a bomb in an already burning town.
In the trial scene, the black people can't enter the courthouse until the white people are inside and, once they do enter, have to sit in the balcony. This literal division of the races was widespread in public places, including movie theaters and opera houses. (That example is only one of several in the novel, of course.)
The 1930s were also squarely in the Depression era, and small details in the novel -- such as Walter Cunningham, Sr. having to consult Atticus about legal difficulties about his land and having to pay for the lawyer's services with labor (e.g. chopped wood) or with produce from his farm (e,g, turnip greens) -- reflect the hardships of small farmers across much of the United States during the Depression.
In the final analysis, though, Harper Lee's novel To Kill a Mockingbird is more than purely a historical account of life in a small town in the 1930s. The novel was published in 1960, and thus it's also very much bound by the social conventions and anxieties of that later time (e.g. the rumblings of Civil Rights Movement).