What exactly is the significance of the Tom Robinson's trial in "To Kil a Mockingbird"? I know what the significance of the trial is roughly about,...

First of all, Tom's trial is tied to the theme of the racism that existed during this time, especially in the South. The fact that Tom is innocent and Atticus proves it shows the extent of the racist attitudes that prevailed throughout American society. The whites on the jury would rather find a black man guilty than to believe that Mayella, a white woman, lied about what Tom did. This also shows the injustice of the justice system. Tom does not get a fair trial, and Atticus knows at the outset that Tom will not be fairly treated. Tom is not judged by a jury of his peers because there are no black people on the jury. Atticus defends Tom, knowing he would most likely lose the case, because Atticus cannot stop trying to get the same justice for blacks that white people enjoy. If no one ever tries to change society, then society will never change. Change usually doesn't happen overnight, and Atticus feels his defense of Tom is a small step toward equal rights and equal justice before the law for blacks.

Tom Robinson and Boo Radley are the characters of the title. Atticus tells Jem and Scout that it's not right to kill a mockingbird because it only brings joy to people with its song and does no harm to anyone. Tom Robinson is one of the mockingbirds of the story, and his conviction and subsequent death are a sin because Tom is innocent.