A reader doesn't ever know what the title means until quite late in "To Kill a Mockingbird." Mockingbirds are never even directly quoted by a character until almost halfway through the book. This is when Atticus says "I'd rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird." This is the first and one of the only times a character directly talks about mockingbirds. There are two main characters that symbolize the mockingbird. Arthur "Boo" Radley and Tom Robinson.
Tom Robinson plays a very important role in the story. Tom teaches the reader as well as many characters in the book very valuable lessons, such as hard work, and equal treatment in the face of adversity. Tom Robinson is symbolic of the mockingbird because all he ever does is good, by helping everywhere he can. Tom helps Mayella do the chores she asks him to out of kindness and never asks for money or expects to be paid. By lying in the courtroom Bob and Mayella Ewell are killing a mockingbird.
Arthur "Boo" Radley is also symbolic of a mockingbird, but in other ways than Tom Robison. Boo stays to himself mostly, but he secretly helps out a lot. He helps the kids lots of times and they didn't ever realize it, when Jem had to go get his pants and they were sewed back, when the kids were outside the house fire and Boo puts a blanket on Scout, and when he starts leaving stuff in the knothole in the tree. The most important time he helps them is at the end of the story when he saves them from Bob Ewell. Boo Radley is innocent and likes to stay isolated, if they would've arrested him for the murder or told people he saved the kids that would have been just like killing a mockingbird. .
The mockingbird itself is symbolic of innocence. Mrs. Maudie said, "Mockingbirds don't do one thing but make music for us to enjoy.