Answer:
Jem was shocked and astonished to hear that one of the jurors was a Cunningham.
Atticus told him that the Cunninghams are good bunch who would be on your side once you earned their respect.
Atticus suggestes that people can change if they know more about the people and the issue. The late night incident at the jail must have changed Mr. Cunningham's view which in turn also led to the stand of the Cunnigham juror siding with Tom.
Explanation:
Jem's reaction to Atticus' revelation that one of Tom's jury was a Cunningham was that of shock and astonishment, added with the fact that he actually asked for an outright acquittal of Tom. He was also rather confused as to why he would do that, <em>"One minute they’re tryin‘ to kill him and the next they’re tryin’ to turn him loose… I’ll never understand those folks as long as I live</em>." (Chapter 23, To Kill A Mockingbird)
Atticus explained to them the Cunninghams are one of the most respectable family who will do what is right irrespective of what the majority people may think or want. And they also keep their word, so if they have respect for someone, that will run till they die, "<em>once you earned their respect they were for you tooth and nail".</em>
Atticus suggested that an individual can be persuaded to change his stance when he mentioned that the night the mob came to the jail to torment Tom, Mr. Cunningham may have felt a sense of respect for the way Scout dealt with him. That night, Scout told Mr. Cunnigham that she attended the same school as his son and that he was a good son. this must have changed his perspective of the Finches for not only did he led the mob to leave, he also seems to have told the Cunningham juror about it for he also changed his stance on Tom's case.
