Who is Jack, the great-grandson from Earth Abides and what is their importance?

Ish becomes aware of Jack one day as the young man shows him an arrow. Ish, it seems, had been greatly unaware of his surroundings for some time but is suddenly interested in this young man who is so eager to talk about any number of topics. It's from Jack that Ish learns that the young people all carry bows and arrows for two reasons - that the guns are too unreliable and that anyone can shoot a gun whereas it takes skill to hunt with a bow and arrow.