OK, so without the force of the preceding paragraphs and chapters, the ending to The Kite Runner is going to sound like one big non-event. This is pretty much what happens: Amir and Sohrab are at a park. Amir gets Sohrab to fly a kite with him, they're getting into it, and Amir vividly remembers Hassan, Ali, and the sounds and smells of Kabul. Sohrab smiles. Also, when Amir asks if Sohrab wants him to chase after the defeated kite, he thinks he sees Sohrab nod. The book ends with a middle-age man sprinting after a kite. This all sounds pretty silly.
However, with earlier events of the novel in mind, it's not so silly. (On a personal note, we also want to justify the tear-fest that happened when Shmoop finished the book.) There's even an outside chance that book-jacket words like redemption and atonement apply here. With traumatic events, what precedes the terrible event often becomes a psychological trigger. This is the case with kite fighting for Amir. Right before he betrays Hassan, Amir participates in a kite-fighting tournament. And so when Amir fights a kite in the park with Sohrab, we half-expect Amir to break down in tears. He doesn't. We think this means Amir goes some way in reclaiming (dare we say "rewriting") his past. As he flies the kite, memories of Kabul come flooding back in a beautiful and moving passage. For most of the book, memory has literally been a nauseating affair, but now it brings joy to Amir.
Still, we're not sure if Amir has really redeemed himself. Sohrab's smile and nod at least suggest redemption is possible. Said another way: Sohrab's gestures suggest it's possible that Amir, by taking in Sohrab, has atoned or made amends for both his betrayal of Hassan and his father's betrayal of Ali. Sohrab has become a sort of stand-in for Hassan (he even looks like Hassan). So, Amir's goodwill toward the boy is a way for Amir to finally do the right thing. Add to this the fact that Sohrab has been utterly silent for a year, that Amir and Baba never talked about their betrayals, and the smallest form of communication suddenly seems really, really important. Sohrab's smile hints at confession and peace after years and years of silence and guilt. That said, Sohrab's gestures are so small and so slight we wonder if Amir really has redeemed himself.