In Chapter 1, the exposition of Harper Lee's seminal novel, a short history of the town is presented, along with the insensitivity of its citizens towards some people such as Mr. Radley and Boo Radley. As Dill and Jem discuss Boo Radley, Dill belittles Jem's lack of daring. Reacting to this insult to his manliness, Jem marches to the corner and warns Dill against the consequences if they try to bring Boo out of his house.
"Dill, you have to think about these things, Jem said. "Lemme think a minute...it's sort of like making a turtle come out."
Here Jem's simile implies that Boo will not want to leave the safety and odd security of his home just as a turtle does not venture out of its shell when it does not feel safe. When extreme measures--setting a match to the turtle's undershell--are suggested as the way to get it out, the compassionate Dill is outraged, telling Jem such an act is hateful. But Jem continues his train of thought regarding coercing Boo out of his house.
Clearly, the turtle is symbolic of the citizens who are not prominent, those who are summarily categorized and dismissed as unimportant. The unconcern for the turtle's feelings by Jem indicate the prevailing unconcern of the citizens of Maycomb for those who are on the fringe of society, hinting at furture chapters and those involved in cruel treatment. In addition, the discussion of Boo's emerging reluctantly from his home, is also a suggestion of his reclusiveness. So, in the latter chapters when Boo emerges on behalf of Jem and Scout, his actions are monumental, indeed.
The treatment of the turtle is a metaphor for Maycomb's treatment of its citizens. The people of Maycomb are judgmental of others without taking the time to understand them.
Jem puts the lighted match under the turtle to coax it from its shell, citing that it represents the way he will coax Boo to come out of his house. Dill, upset by Jem's mistreatment of the turtle, tells him it's a hateful thing to do. Jem responds that it's alright because turtles can't feel. Dill then questions whether Jem has ever been a turtle. In other words, don't judge something you've no understanding of.
To Kill A Mockingbird/ Chapter 1