The crowd is obviously on pins and needles. Everyone knows that Tom is innocent and it shows with their body language (one reason why there is such attention given to detail) and the gasps they let escape at certain times. One of those times is when Atticus brings attention to the fact that the marks left on Mayella's body had to have been left by a left-handed attacker and then he asks Tom to show the jury and everyone in the courtroom his left arm. It is obvious to everyone that he couldn't have been the attacker based on the evidence since his left arm is badly mangled as a result of a farming accident in his youth.
Tom also remarks at one time in the testimony that he felt sorry for Mayella. The affronts the white men in the audience as they feel Tom, a black man, has overreached his social station. It is not his place to feel sorry for a white woman regardless of her sitauation...in their opinion.
The white people sit downstairs and are looking for a guilty verdict simply because of Tom Robinson's color.
The black people sit upstairs and are hoping but are not expecting the truth to be seen and for Tom to go free. Even though the Ewells are "trash," they are still white, and the black community realizes that their word is still considered more credible than a black man's word...even from such a respected and well-liked black man as Tom.
The trial is an important stepping stone in the coming of age of Jem.
In the old South, "lynch mobs" whose intent was to hang "negroes" were prevalent. This crowd is much the same, longing for a guilty verdict like wolves to the kill. The author describes them in an anxious and eager way because that is exactly what they are. Their descriptors also include factors such as their lifestyles and appearances: agrarian and rural.