explain what happen when tom robinson was moved to the county jail

Sheriff Tate and a group of other men come by the house to tell Atticus that Tom Robinson is being moved to the county jail and that there may be trouble. That Sunday night, Atticus heads into town, which gives Jem a funny feeling.

At bedtime, he, Scout, and Dill walk downtown themselves to see what's happening. They find Atticus sitting outside Tom Robinson's cell and turn to head home when a group of men arrive to confront Atticus. Not realizing the danger of the situation, Scout runs into the middle of the mob. After a few tense moments, she begins a conversation with Walter Cunningham's father, which causes the men to retreat, and very likely saves Atticus' life.

The next morning, the day the trial is set to begin, Atticus and Scout talk about mob mentality, and, over Aunt Alexandra's protests, he thanks the children for appearing when they did. He asks the children to stay away from the courthouse during the trial, but by noon, their curiosity has the better of them, and they, along with Dill, head for the courthouse where the trial is about to get underway. They can't find a seat in the courtroom, so Reverend Skyes offers them seats in "the Colored balcony," which they gladly accept. Finally, readers are introduced to Judge Taylor, who the children earlier discovered — much to their surprise — appointed Atticus to defend Tom Robinson.