Atticus tells Scout what about most people?

I believe that this question is asking about the final lines of the entire book.  Atticus is putting Jem and Scout to bed, and Scout wants Atticus to read a story out loud.  Atticus agrees to read a story, and Scout falls asleep.  As Atticus is finally putting Scout to bed, she wakes up enough to say that she heard every word of the story.  To Scout's credit, she gives a decent summary.  She explains how everybody thought it was "Stoner's Boy" messing everything up.  The people were angry at the boy until they finally saw him and realized that he didn't do anything.   

Yeah, an‘ they all thought it was Stoner’s Boy messin’ up their clubhouse an‘ throwin’ ink all over it an‘…
[...]
An‘ they chased him ’n‘ never could catch him ’cause they didn’t know what he looked like, an‘ Atticus, when they finally saw him, why he hadn’t done any of those things…

Scout ends by saying that the boy was actually "real nice."  Atticus responds by telling Scout that most people are real nice.  

Most people are, Scout, when you finally see them.

It's a great close to this story because readers got to see Scout learn this lesson after having met Boo Radley. 

He had to stoop a little to accommodate me, but if Miss Stephanie Crawford was watching from her upstairs window, she would see Arthur Radley escorting me down the sidewalk, as any gentleman would do.