You have to compare to teacher to something else like: "My teacher is as sneaky as a fox" ; Something like that.
It's a source citation. The name Breenan is the author, and 142 is probably the page this information was found on in Breenan's writing.
The purpose is because some one must of sayed that in a story or a title so you don't want to take what they sayed it's like steeling
Side stories of other people add to the story , because it shows how they feel about the topic and sooner in the story you'll see how boo radley and tom Robinson are similar to each other