The mockingbird in this novel is a symbol for natural innocence and goodness. They are creatures which 'don't do one thing but sing their hearts out for us,' as Miss Maudie observes.
Tom Robinson is an example of an innocent, good-hearted, wholly inoffensive man who is made to suffer by society. He falls victim to the racial prejudice that is endemic in a town like Maycomb, in the American South of the 1930s. This racism is both casual and institutionalized. When he is accused of molesting a white girl, Mayella Ewell, and put on trial, the evidence points strongly to the girl's own father having attacked her, and not Tom. Yet Tom is the one to suffer, simply because he is black; black people are regarded as second-class citizens (and worse) in this society, and Tom pays for another's crime. Tom had only wanted to help Mayella - Scout realizes vividly during the trial that 'Tom was probably the only person that was ever kind to her' - yet he ends up accused of rape.
Scout and Jem are devastated when Tom is convicted, although their father Atticus, a man with an eminent sense of fair play, fought valiantly for him as his defence counsel. The children simply cannot understand at first why Tom is so unfairly convicted, until they realize that the only factor that ultimately matters in his case is his colour. The general virulent racism of the town ensures that Tom cannot be acquitted, despite the efforts of more enlightened individuals like Atticus (and also Judge Tate) because the all-white jury is simply unable to look past Tom's colour. Scout comes to a stark understanding of the fact that, despite the whole apparatus of a trial, Tom really had no chance at all:
.... in the secret court of men's hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed.(chapter 25)
Tom, then, is a wholly innocent man who only tried to help a white woman - an action most willfully misconstrued by a racist society. After he dies when trying to escape jail, Mr Underwood, a local editor, likens his death to 'the senseless slaughter of songbirds' - thus emphasizing his connection with the mockingbird, the symbol of innocence.