There are several themes shown in Chapter 15 of Harper Lee's To Kill a Mockingbird.
A theme is a message, or life truth, which the author is trying to share with the reader, by way of a piece of literature. In this novel, there are a number of themes. Among them are "prejudice and tolerance," "knowledge and ignorance," "courage and cowardice," and a man's humanity—regardless of color.
The chapter begins when men from town come to Atticus' home—worried about Tom Robinson. In them we see the positive aspects of each of these themes: tolerance, knowledge, courage, and humanity.
The men come to Atticus's home—this indicates forthrightness about their actions. They do not sneak, but meet Atticus in a civilized manner. They are honest and courageous enough to speak to Atticus directly. And while there may be tension in the discussion, there is no danger.
"...moving him to the county jail tomorrow," Mr. Tate was saying. "I don't look for any trouble, but I can't guarantee there won't be any..."
"Don't be foolish, Heck," Atticus said. "This is Maycomb."
"...said I was just uneasy."
"Heck, we've gotten one postponement of this case just to make sure there's nothing to be uneasy about...You can keep him one night, can't you?"...
Mr. Link Deas said, "Nobody around here's up to anything, it's that Old Sarum bunch I'm worried about...can't you get a—what is it, Heck?"
"Change of venue," said Mr. Tate...
...Atticus was saying, "you're not scared of that crowd, are you?"
Here we see the concern of Atticus' tolerant, respectful and honest neighbors of Maycomb. They do not let their personal feelings about Tom Robinson's race or alleged crime interfere with justice being served. They are taking precautions for his safety.
...they were people we saw every day: merchants, in-town farmers; Dr. Reynolds was there; so was Mr. Avery.
This group is very different than the one that visits Atticus at the jail. The mob shows up ready lynch Tom on the spot. As the men at Atticus' house had worried, here are men with no sense of justice—actions are fed by a mob's mentality—emotions are running high: men's minds made up on the spot.
...four dusty cars came in from the Meridian highway, moving slowly in a line...
Nobody got out. We saw Atticus look up from his newspaper...He seemed to be expecting them...
In ones and twos, men got out of the cars...lights revealed solid shapes moving toward the jail door...
"He in there, Mr. Finch?" a man said.
"He is," we heard Atticus answer, "and he's asleep. Don't wake him."
In obedience to my father, there followed what I later realized was a sickeningly comic aspect of an unfunny situation: the men talked in near-whispers.
"You know what we want," another man said. "Get aside from the door, Mr. Finch."
"You can turn around and go home again, Walter," Atticus said pleasantly."
The group has purposely distracted Mr. Tate, and has come to take Tom. They behave just the opposite of the men who met Atticus on his lawn. They are prejudiced, ignorant and cowardly. They have judged Tom guilty without a trial first because he is a black man accused by a white man, and also because they haven't thought about their actions. This does not happen until Scout interrupts with questions about Walter's family and life. She is able to get Walter to mentally step away from the mob—and to consider what he's doing—to recognize his humanity, and that of others: the Finches'...and perhaps the Robinson's.
Virtually all of the main themes of TKAM can be found in this chapter. One of the most obvious is that of courage and cowardice. Atticus illustrates to both his children and the lynch mob that courage is not necessarily displayed by a man "with a gun in his hand." Atticus risks his own life to defend his client, and in the end, the cowardly lynch mob is shamed into leaving by the equally courageous actions of Jem and Scout (and Dill), who stand by Atticus in spite of their lack of understanding of the events that are transpiring before them. Jem displays bravery by standing up to his father and refusing to leave his side; Scout shows her toughness in a moment of levity when she defends Jem, attacking the man who
... yanked Jem nearly off his feet.
I kicked the man swiftly. Barefooted, I was surprised to see him fall back in real pain. I intended to kick his shin, but aimed too high. (Chapter 15)
The newspaper editor, B. B. Underwood, no lover of Negroes, stands guard from above with a shotgun in his hand, ready to defend Atticus if things get too serious.
The theme of loss of innocence reoccurs here as the guiltless Tom Robinson is nearly taken out to be hanged. The three children don't realize the gravity of the situation until the next day; it is only then that Scout realizes all of their lives were in danger.
The full meaning of the night's events hit me and I began crying. (Chapter 16)
The theme of guilt and innocence is also present as the lynch mob decides that committing murder is the proper punishment for Tom, who has been charged with crimes he did not commit. And certainly the themes of racial prejudice and tolerance are visible: The all-white lynch mob is only interested in killing Tom because he is black; Underwood, an avowed racist himself--"He despises Negroes. Won't have one near him"--nevertheless allows his conscience to rule his actions; and Scout remembers that it is "the polite thing to talk to people about what they were interested in, not about what you were interested in." Her talk with Cunningham brings him to his senses, and the mob leaves quietly.