As the jury is deliberating on the evidence in the trial of Tom Robinson, it is taking a pretty long time. As Scout sits in the courthouse waiting, she says that the strangest feeling came over her, the same exact feeling that she got as she was watching the rabid dog Tim Johnson come down the street, waiting to be shot. Everyone on the street knew he was there, but shut their doors tight against him, and they knew that Atticus was out there waiting the help the situation, but in fear they stayed inside. Waiting for the dog to come down the street and meet his end was suspenseful, and had a surreal quality to it. This is what Scout thinks of as she is waiting for the verdict to arrive. She says the entire courthouse is full of people, just like the street was, but that everyone was perfectly quiet and tense, just like they were that day with the dog also. They all knew something important, something huge was going to happen, that was going to change someone's life.
The comparisons in symbols are quite a few. Lee could be saying that Tom Robinson had no chance, just like the poor rabid dog had no chance, and that their southern society viewed Tom no differently than that dog. The suspense was a suspense of people waiting to hear what they knew would happen--a death sentence, or at least a sentence akin to a death sentence--to be pronounced, just like with the dog. Using that eerie mood of waiting sets the stage for the courtroom scene quite well, no matter the meaning, and that is how Lee chooses to invoke that feeling of eerie suspense.
I hope that helped; good luck!