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. 

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.

What did Rahim Khan mean By "there's a way to be good again"? Why is it important for Amir to redeem himself? If so to whom and why?

Amir, in his childhood, behaved in ways that caused grievous wrongs to Hassan and Hassan's father. In fact, it could be reasonably said that his behavior played a part in their deaths.

While Amir is generally arrogant and domineering to Hassan when they are children, the first really striking wrong we see in the novel is when Amir hides in an alley while a group of bullies sodomize Hassan.

Since the presence of Hassan in the household is a constant reminder to Amir of his own wrongdoing, he manipulates events so that Hassan is accused of a theft, and that causes Amir's father, Baba,  to "exile" Hassan and Hassan's father, Ali,  from the household.

Because of the subsequent upheaval in Afghanistan, Amir and Baba are forced to flee the country. Had Hassan and Ali remained in the household, there is a good chance they would have left for America with Amir and Baba.  Both must remain and both die.

A chance to be good again is a chance for redemption, which is a prevailing theme of the novel.  While Amir and Ali have died, Amir has left a son behind, Sorhab, who will have no chance for a good life unless Amir can rescue him.  Such a rescue is a way for Amir to overcome his own cowardice, atone for his sins against the family, to redeem himself in his own eyes, and to save one child from a terrible life or death.

You will notice that the book opens with the idea of redemption and closes with the redemption, telling the reader what a powerful and important idea this is in the story.