This represents Charles Baker Harris (Dill) arriving in Chapter 1. It is an important part of the book because Dill's character is an extremely important part of the story.
It aint my fault Uncle Atticus is a nigger lover.
Dill is daring Jem to go knock on the on the Radley's door knowing Jem never backs out of a dare. Doing everything in his power to not do it, Jem ends up doing it anyways.
... remember its a sin to kill a mockingbird.
In Chapter 7 Miss Maudie's house catches fire and burns down, Atticus awakes Scout and tells her to put her bathrobe on and come out, through all the confusion and noise a blanket gets dropped over her, not knowing by whom. Jem later realizes it was Boo Radley.
Your daddy is a nigger lover.
In chapter 9, Atticus takes Jem and Scout to Finch's landing where his sister and her husband live, which is where Francis, Atticus's sisters grandson is, he keeps calling Atticus names which made Scout mad, and she fights him.
This is important because this references to the title, and what he means is mockingbirds are innocent, all they do is sing for us. Hes saying that innocent people get convicted for nothing all the time. AKA Tom Robinson
This is important because its a turning point in the story, Jem snaps at Mrs. Dubose' comment and tears apart her bushes. His punishment is he has to read to her, while doing that she passes away.