It is the idea of treating someone as an individual, which is the most important part of this event in terms of the rest of the novel.
The mob appears at the courthouse in chapter 15. We have seen an appeal for justice at the beginning of the chapter, as Dill ‘the defendant’ has been allowed to stay in Maycomb. This is of course, not a legal decision, but a judgment nonetheless, and one which takes in the individual ( in this case Dill) and the particular circumstances.
The mob arrive at the courthouse to exact their own justice as they see it. Their ideas are not part of legal precedent: they are vigilantes seeking to avenge the abuse of a young woman. Mr. Cunnungham is a simple man, and has lived by simple rules. He keeps his dignity with Atticus by paying his bills in kind rather than cash. Being part of the mob, he loses his identity and temporarily forgets the importance of the individual and the compassion which should accompany the treatment of each person. When the mob first form, and Jem is afraid they will hurt Atticus, it is Atticus who refuses to see them as such –
No, son, those were our friends.
Scout brings Mr Cunningham back to himself by discussing his ‘entailment’ and inferring that groups should solve problems rather than create them –
…it takes a long time sometimes…that you’d all ride it out together.