At first I thought that motif was just another one of those terms we would have to know for the occasional vocab test. I didn't think it would actually be used a ton in novels, but I was very surprised. In To Kill a Mockingbird it is used to describe characters, such as Boo Radley or Tom Robinson. It also helps show themes such as racism.
A mockingbird is an innocent bird that just sings songs. When Jem and Scout get their guns, Atticus and Ms. Maudie tell them that it a sin to kill the birds, because they are just happy and cheerful. This is used as a motif and helps show the theme of racism. In the Tom Robinson case, Tom is clearly proven not guilty, so he is like an innocent mockingbird. All 9 jury members voted Tom to be guilty, giving him the death penalty, so that is like killing a mockingbird. The jury members were racist against blacks, so that is why they voted him to be guilty. Several other themes that the mockingbird motif is present in is justice vs. injustice, prejudice, and compassion. The motif of killing a mockingbird is used in compassion with Boo Radly when Heck Tate says that Mr. Ewell fell on his knife, not that Boo stabbed him. This is compassionate because he sees the mockingbird in Boo and wants to let him do whatever he wants.