This scene actually begins on Saturday evening, one day before the scene at the Maycomb jail. Scout and Jem are intrigued when a group of men gather on their lawn to speak with Atticus. They turn out the lights so as not to be seen and eavesdrop on the conversation outside. Several mean they know, including Heck Tate and Link Deas, are concerned that the Old Sarum bunch are going to try to interfere with the trial by going after Tom before the trial even begins. In chapter one, we learn that the Cunningham family is part of this group and that "they formed the nearest thing to a gang ever seen in Maycomb."
Atticus remains calm, saying that the next day is Sunday and that those men don't drink on Sundays. Scout is surprised to see many of these same men who were in her yard on Saturday show up at church on Sunday; it seems that they want to continue this conversation with Atticus and Scout believes that "something must have been up" to summon Mr. Underwood's church attendance in particular.
Thus, there is a mood of suspicion as Sunday afternoon passes. When Atticus leaves that evening carrying an extension cord and a light bulb, Scout and Jem pay attention. When he decides to drive his car instead of walk, as is typical, Jem gets a feeling that something is wrong.
Jem and Scout sneak out of the house, picking up Dill along the way, and set out to find their father. He isn't in his office, which is suspicious, and they spot a light shining at the Maycomb jail; this is odd because the jail doesn't have an outside light.
When the children get closer to the jail, they can hear the voices of men, talking in near-whispers. Through the darkness, the men demand that Atticus get out of their way. They have come to lynch Tom Robinson, which they allude to vaguely when they tell Atticus, "You know what we want." Atticus refuses to move, believing that Heck Tate will return to support his efforts. When the Old Sarum gang tells Atticus that they have sent Heck and his bunch on a "snipe hunt," or an impossible mission, it looks as if the group is about to turn violent toward Atticus.
Scout emerges from the darkness, with Jem and Dill close behind her. She is able to deescalate the situation by reminding Walter Cunningham, Sr. how her father has helped him out in the past and by mentioning his own son, whom Scout describes as "real nice" and a "good boy." Scout's innocent reminders that Atticus is not the enemy of the Cunninghams is enough to send everyone home that evening.