what is the reason behind the naming of the book to kill a mockingbird?

Answer 1

I can't put it any better than how it is described in Sparknotes. I normally don't like cutting and pasting - so apologies, but this is good,

"The title of To Kill a Mockingbird has very little literal connection to the plot, but it carries a great deal of symbolic weight in the book. In this story of innocents destroyed by evil, the “mockingbird” comes to represent the idea of innocence. Thus, to kill a mockingbird is to destroy innocence. Throughout the book, a number of characters (Jem, Tom Robinson, Dill, Boo Radley, Mr. Raymond) can be identified as mockingbirds—innocents who have been injured or destroyed through contact with evil. This connection between the novel’s title and its main theme is made explicit several times in the novel: after Tom Robinson is shot, Mr. Underwood compares his death to “the senseless slaughter of songbirds,” and at the end of the book Scout thinks that hurting Boo Radley would be like “shootin’ a mockingbird.” Most important, Miss Maudie explains to Scout: “Mockingbirds don’t do one thing but . . . sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” That Jem and Scout’s last name is Finch (another type of small bird) indicates that they are particularly vulnerable in the racist world of Maycomb, which often treats the fragile innocence of childhood harshly."

Answer 2

“Remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
“Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”

These lines from Chapter 10 are the source of the novel’s title and introduce one of the key metaphors of the book: the idea of “mockingbirds” as good, innocent people who are destroyed by evil. Boo Radley, for instance, is like a mockingbird—just as mockingbirds do not harm people but only “sing their hearts out for us,” Boo does not harm anyone; instead, he leaves Jem and Scout presents, covers Scout with a blanket during the fire, and eventually saves the children from Bob Ewell. Despite the pureness of his heart, however, Boo has been damaged by an abusive father. The connection between songbirds and innocents is made explicitly several times in the book: in Chapter 25, Mr. Underwood likens Tom Robinson’s death to “the senseless slaughter of songbirds by hunters and children”; in Chapter 30, Scout tells Atticus that hurting Boo Radley would be “sort of like shootin’ a mockingbird.” The moral imperative to protect the vulnerable governs Atticus’s decision to take Tom’s case, just as it leads Jem to protect the roly-poly bug from Scout’s hand.


have a nice day.

Answer 3

Easy!
You'll find out by reading the book.