What did Rahim Khan mean when he said to Amir, "there is a way to be good again?"
One of the main themes of this novel is redemption. When Amir and his father fled Kabul and went to the United States there was a shadow of shame over Amir. In "The Kite Runner" Amir had shamed himself by not coming forward and helping Hassan when he was attacked. Amir shamed himself even further by lying when asked if he knew what was wrong with Hassan and further, he created a situation in which Hassan was accused of stealing and had to leave his home. Amir was so determined to win his father's love for himself and only himself that he hurt Hassan in the process. His years from that point on were filled with guilt. He needed redemption.
When Rahim called Amir and told Amir that he needed to come back home Amir was unsure. Rahim told him that he needed to come to right the wrongs that had been done, that "there is a way to be good again" indicated to Amir that there was an action he could take to redeem himself from the wrongs he had committed in his youth.
The reader later learns that Hassan was his half brother and that he had had a son. Amir could make things good again by saving his nephew.
Rahim Khan essentially means that there is a way for Amir to atone for his past sins and redeem himself by telling him that there is a way to be good again.
Rahim Khan knows that Amir witnessed Hassan being raped as a child and did not intervene. Rahim also understands that Amir has been living with the guilt of his decision for decades. When Rahim Khan calls Amir from Pakistan and informs him of his present physical condition, he is confident that Amir will visit him. Rahim Khan also knows that Hassan is Amir's half-brother, which is something that Amir does not know. Rahim's plan is to tell Amir the truth about Hassan being his half-brother, then inform him of Sohrab's present circumstance in hopes that Amir will travel into Taliban-occupied Kabul to save Hassan's son. Fortunately, Amir ends up atoning for his past sins by saving Sohrab, adopting him, and bringing him to America.