At the end of the novel, and in response to Scout's summary of The Gray Ghost, Atticus remarks that most people are good "when you finally see them." In this context, most readers connect this statement with Scout's perception of Boo Radley--the mysterious neighbor who saved Scout's life (and who turned out NOT to be as scary as the children had thought). However, it is evident, from other information in the text, that the belief that all people are basically good is one that Atticus holds firmly.
A perfect example comes in Chapter 15, when Atticus is accosted by a mob outside the jail where Tom Robinson is being kept. At the center of the mob is Walter Cunningham, Atticus's client and the father of Scout's schoolmate. It becomes clear that the men have come to kill Tom Robinson, and Scout's innocence is the only thing that seems to be able to stop them.
The children are shocked by the fact that Mr. Cunningham seemed willing to hurt Atticus in order to get to Tom, and Jem says, "I thought Mr. Cunningham was a friend of ours...But last night he wanted to hurt you." In response, Atticus says,
Mr. Cunningham's basically a good man. He just has his blind spots along with the rest of us.
When Jem is unwilling to accept this explanation, Atticus says,
Son, you'll understand folks a little better when you're older. A mob's always made up of people, no matter what. Mr. Cunningham was part of a mob last night, but he was still a man. Every mob in every little Southern town is always made up of people you know--doesn't say much for them, does it?
In other words, Atticus understands the strength that comes from being in a group, and is generally forgiving of Mr. Cunningham's behavior because Mr. Cunningham is ultimately a good man who just got caught up in the mob mentality.
Ultimately, Atticus's lesson to his children is that "you can never really understand a person until you consider things from his point of view--until you climb into his skin and walk around in it." Recognizing the good in people is what makes Atticus so tolerant of their "blind spots."