The title of the novel seems appropriate, since it comes from one of Atticus's most important talks with his children. His explanation that "it's a sin to kill a mockingbird" is more than just a reminder not to kill the innocent songbird, but it is also symbolic. It is also a sin to take advantage of other people--such as black people and Boo Radley. The death of Tom Robinson--one of the novel's human mockingbirds--is also related to the title.
As for being outside the "social circle," there are many different social levels in Maycomb. Jem believes there are four different types of people--
"... the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the wood, the kind like the Ewells down at the dump, and the Negroes."
Jem may not be entirely on target, but his four kinds of people do live and associate in different social circles. The Cunninghams are rural folks who primarily live in Old Sarum in the northern part of the county. The Negroes live in the Quarters outside the southern town limits, and the Ewells fittingly live in between Maycomb and the Quarters adjacent to the dump. None of these people seem to socialize with one another, and most are not welcome in the others' areas. Segregation prevents whites and blacks from socializing or even sharing many public places; and some people, such as Aunt Alexandra, believes many of her non-neighbors are simply "trash," such as Walter Cunningham Jr. And then there is Dolphus Raymond, a white man who lives with a black mistress, who is not totally welcome in either the black or white worlds.