Tom Robinson is symbolically linked to a mockingbird. As Atticus tells his children, "It's a sin to kill a mockingbird because they don't hurt anyone."
Tom doesn't hurt anyone, even though he is falsely accused of raping Mayella. He is killed physically trying to escape from prison, but he is also "killed" symbolically when his character is besmirched by this accusation. He is unable to work to support his family and, for all intents and purposes, is dead to the community.
Tom Robinson is symbolically linked to a mockingbird. Remember, Jem and Scout are warned not to shoot at mockingbirds, for they do not harm anyone. They just sing and are beautiful to behold. This best resembles Tom. He supports his family and helps others in need (he even helped out Mayella Ewell). However, when he is wrongly accused, only a handful of Maycomb's citizens step up to defend him. Because of the racism inherent in the town and the majority of its citizens, Tom is found guilty. He is sent to prison where he is shot trying to escape. The town sinned by convicting an innocent man just because of the color of his skin.