Scout and Jem get air-rifles for Christmas in chapter 9. Then in chapter 10, Scout mentions that Atticus tells Jem the following one day:

"I'd rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird" (90).

Scout asks Miss Maudie about this and is told that mockingbirds are not pests like bluejays are. Mockingbirds provide beautiful music for everyone to enjoy, in fact. Also, mockingbirds are innocent, vulnerable, and don't hurt anyone or anything; therefore, Maudie backs up what Atticus says. But there certainly is a symbolic and parallel meaning between mockingbirds and Tom Robinson. Tom is an innocent man with a wife and children; he goes to work faithfully each day; and he is kind enough to help out a young girl named Mayella Ewell a few times by chopping wood for her. As a result, he gets caught in her vixen trap as she tries to kiss him one day. Her father, Bob Ewell sees this and presses rape charges! Poor Tom is like a mockingbird because he never hurt anyone, yet he dies because of the discriminatory and racist social system in the South.