Atticus stood guard at the jail so that the local people would not lynch the prisoner that was inside. Atticus felt he needed a proper trial, which did take place, and the man was found innocent.
Tom is in the Maycomb County Jail during the time that Atticus faces the mob downtown.
a light
are you referring to when the mob approaches Atticus at the county jail? If so, it's because they have come to lynch Tom Robinson, who is in the jail. I believe Atticus suspected this, and decided to go sit in front of the jail to protect Tom.
The Children finds Atticus in front of the maycomb jail.
He was sitting outside of the county jail to prevent a lynch mob from attacking Tom Robinson.
A man like Mr. Underwood was covering Atticus at the jail because Mr. Underwood knows that he was innocent.
Scout and Jem carry an office chair and a light to the jail in "To Kill a Mockingbird" in order to sit and provide light for their father, Atticus, who is guarding the jail.
Jem didn't do anything, he just brought scout to the jail while the mob was there. Scout showed the mob that Atticus helps everyone. The mob wanted Tom Robinson, but they would have to go through Atticus (hurting him in the process most likely) to get to Tom. If Jem didn't bring Scout, Atticus would have been injured in some way.
Dill shows respect for Atticus by understanding the gravity of the situation outside the jail and acknowledging Atticus's bravery and integrity in the face of danger. He also looks up to Atticus as a role model for standing up for what is right and remaining composed under pressure.
I'm assuming this is about To Kill A Mockingbird. I just finished this in class. In those days, it was common for mobs to go and lynch (or kill) a black man before his trial. It was easier in their eyes, and the local government just turned a blind eye. Atticus went there to protect Tom Robinson from the mob that inevitably came.
Atticus knows Tom Robinson is coming back to the Maycomb jail because he received a tip from Heck Tate, the sheriff, that a mob might try to harm Tom. Therefore, Atticus decides to sit by Tom in an attempt to protect him.
Atticus remains calm and composed when Scout runs to him at the jail, despite the tense situation with the mob. He is relieved to see her but also tries to protect her by keeping her away from the dangerous situation unfolding.