There are actually two. Tom Robinson is like a mockingbird. He has done nothing wrong, yet he is found guilty of a crime he did not commit and sentenced to prison where he dies. It is a sin that this was allowed to happen.
Boo Radley can also be viewed as a mockingbird. Scout observes, after overhearing Heck Tate and Atticus discuss what to do about Bob Ewell's murder, that putting Boo through a trial would be like killing a mockingbird. Boo has done nothing wrong but silently observe - and once in awhile interact - with the children. When they needed his protection, he didn't hesitate to protect them. Putting this shy, reclusive man on trial for saving Jem and Scout would be like killing a mockingbird. Who knows what kind of effect it would have on him? So Heck Tate states that Bob Ewell simply fell on his knife and Scout walks Boo home.