Atticus reminds Scout she should call him Mr. Arthur, and then he formally introduces them to each other.
Scout plays hostess to Boo, showing him the way and offering him a chair, though the whole thing feels not entirely real.
And then Atticus and Tate start talking about Jem's court case.
Apparently, Atticus thinks that Jem killed Ewell, but Tate thinks that's ridiculous and has some story about Ewell falling on his own knife and impaling himself.
It's clear that Tate's trying to cover something up, but Atticus—who's just too honest for his own good—can't quite figure it out.
No one ever quite says it, but the point is that Boo killed Ewell, and Tate just doesn't want to drag Boo through that publicity, both the good (no one really liked Ewell) and the bad—like, you know, trial for murder, or at least manslaughter.
When Tate leaves, Atticus asks Scout if she understands.
Yeah, says Scout: it would be (title alert) like killing a mockingbird.