The aptly named To Kill a Mockingbird refers to the advice that Atticus gives Jem and then is repeated by Miss Maudie to Scout in Chapter 10.
"Your father's right," she said. "Mockingbirds don't do one thing but make music for us to enjoy... they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
The mockingbird is the symbol of innocence throughout the novel. Several of the characters--Tom Robinson, Boo Radley and most of the children--also fit Maudie's description and become the human mockingbirds of the story. They are innocent victims of a cruel world. Tom is sentenced to prison despite his innocence. Boo is the subject of cruel gossip that is later proven to be unfound. Mr. Underwood's editorial compares Tom's death with that of killing a "songbird." The children--particularly Jem, Scout and Dill--are innocent young victims who witness and suffer the realities of a sometimes harsh world. They eventually become the targets of hatred and indifference--much like the mockingbird.