The author makes it clear that, though often difficult and possibly a lengthy process, a man can atone for his past sins. Amir's guilt over his sins against Hassan--standing idly by while he is raped, and then planting evidence that would seem to make him guilty of theft--hang over him for more than 20 years. It affects his relationship with his father and his wife. But when he returns to Pakistan to visit Rahim Khan and discovers the truth about Hassan--that he is actually the half-brother to Amir, and he has left behind a son--Amir determines that this is his chance to eradicate the guilt that plagues him. Rescuing Sohrab from the Taliban and bringing him home to be a part of his family begins the atonement for his past indiscretions. When Amir volunteers to run the kite for Sohrab--acting out a reversal of roles that Amir and Hassan shared as youths--he solidifies the redemption process.
One of the main themes is obviously guilt, particularly the guilt that Amir feels ever since that day in his childhood when he allowed Hassan to be raped after running his kite for him. That guilt is only intensified when he learns that he and Hassan are actually brothers.
But the great power of redemption and its importance is also shown by Amir's trip to Afghanistan and his standing up to the very same bully that he backed down from in his childhood. The redemption he feels is important and notable because it isn't as though he can pay back Hassan directly but by helping his son he feels that he has repaid some of his debt.