Both Tom Robinson and Boo Radley serve as symbols of mockingbirds. However, another interesting way to look at this is from Boo's point of view. In the final chapter, Scout walks Boo home and pauses by his window. She envisions what the world must have looked like from his vantage point. She recounts watching the kids play and grow. Then when they needed him (when they are attacked by Bob Ewell), Boo protects them.
If you think about it, Jem could serve as a mockingbird symbol too. He has done nothing but strive to help Boo (trying to get him to come out and realize they mean him no harm and only want to befriend him) and feel compassion for him (notice how hard he takes it when the knot in the tree is filled with cement and Boo is cut off from communicating with them or how he cries when he realizes Boo has mended his pants when he was trying to sneak Boo a note). So from Boo's point of view, it would be a terrible sin to harm Jem since he has done nothing but provide - albeit mostly vicariously - entertainment and friendship.
When Boo rushes to defend the children - and kills Mr. Ewell - he is bringing an end to the repercussions of the Robinson trial. Remember, Mr. Ewell was seeking revenge for how Atticus humiliated him on the stand. Boo also brings a close to the mystery surrounding him in the novel too.
The idea behind the mockingbird is what Atticus tells his kids: It's a sin to kill a mockingbird because the mockingbird doesn't do anything to hurt other birds. Arthur (Boo) Radley and Tom Robinson both represent the mockingbird. People are afraid of Boo and make up stories about him because they don't understand his illness, despite the fact that he stays in his house and doesn't bother anyone. Tom Robinson is on trial for a crime he didn't commit and doesn't have a chance of being acquitted because he is black and is accused of raping a white woman.
Check the links below for more information on the themes and characters of this wonderful book. Good luck!