Chapter 15 of To Kill a Mockingbird finds Atticus having a meeting with Heck Tate and other members of the community. They have come to the house to discuss the fact that Tom Robinson is being moved to the county jail. The men tell Atticus of their fears of there being some sort of violence at the jail. Atticus doesn't voice his concerns to Jem and Scout, but after church and dinner, Atticus takes the car and leaves. Jem is worried about his father, so he and Scout decide to go and see their father.
A long extension cord ran between the bars of a second-floor window and down the side of the building. In the light from its bare bulb, Atticus was sitting propped against the front door. He was sitting in one of his office chairs, and he was reading, oblivious of the nightbugs dancing over his head.
Atticus was there to prevent any fights from breaking out. He knew that Bob Ewell and his friends would come and start trouble, even try to kill Tom, so Atticus was there keeping watch. True to fashion, Bob does show up, but thanks to the innocence of Scout, nothing bad happens that night.
Atticus is approached at home that evening by a group of men who feel uneasy about having Tom Robinson, a black man, in the Maycomb jail. Atticus assures them that Tom will remain in the jail until the trial.
Atticus, being the intelligent man he is, knows that Tom may be in danger. So, Atticus gets a chair, a light and reading material, and positions himself in front of the jail in order to protect Tom, in the event that townspeople would come to hurt him.
It turns out that Atticus' suspicions were correct. A group of men does show up at the jail to take Tom Robinson. Atticus, along with Jem and Scout, save Tom's life by being there that evening.
He was protecting Tom Robinson from the mob