The answer to your question is in Chapter 10 when Miss Maudie explains what Atticus said to Scout about killing a mockingbird. Basically, mockingbirds do no harm, only bring joy and happiness. Tom is a very helpful man who takes time out of his very busy work schedule to help Mayella Ewell for whom he feels pity. All characterization of Tom shows him to be kind, gentle, and good--Scout even pays him the highest compliment by saying he is very much like Atticus. All of Tom's deeds are to help someone he believes is lonely and without much assistance from her father and the other children. Because he is accused of a capital crime, and is guilty because of the color of his skin, Tom's death is a true sin fueled by hatred and prejudice. Tom is the best example of a mockingbird because he is wrongfully murdered. The tragedy of Tom's death's is doubled because of the irony of his arm: the one piece of evidence that should have freed him--his withered arm--succeeds in being the reason he dies.
Tom Robinson is the accused man in the novel To Kill a Mockingbird. He is poor and black, making his rape of a white woman an outrage in this 1930s southern Alabama town. His lone defender is Atticus Finch.
Beyond this literal characterization, Tom Robinson is a common figure for racism. By nature, Tom was kind, caring, and mild-mannered. He helped Mayella Ewell frequently when she was left to do chores alone because he felt sorry for her. The fact that a black man felt sorry for a white woman was offensive to the white people of the time who could not understand how any black person could feel "better" than a white person.
Tom is also a symbol of injustice, especially racial injustice. He was clearly innocent. His useless hand and arm and other indisputable facts proved him so, but that did not matter to the jury of white men who convicted him.
Finally, Tom can be seen as a martyr. He was innocent, but doomed to die for the crime. Not willing to do so, he escapes jail and is shot dead.