What does the sour apples story say about Sohrab?

It means that he has learned to have patience;

One time, when I was really little, I climbed a tree and ate these green, sour apples. My stomach swelled and became hard like a drum, it hurt a lot. Mother said that if I'd just waited for the apples to ripen, I wouldn't have become sick. So now, whenever I really want something, I try to remember what she said about the apples.