At the end of Chapter 3, Amir overhears his father speaking about him to Rahim Khan. Baba tells Rahim Khan that he was not like Amir growing up and notices how the other neighborhood boys push him around. Baba is disappointed at the fact that Amir never stands up for himself by fighting back. Baba says that he watches how Hassan constantly defends Amir and believes that something is missing in his son. After Rahim Khan tells Baba that he needs to let Amir find his way, Baba says,

"A boy who won't stand up for himself becomes a man who can't stand up to anything" (Hosseini 20).

This quote not only describes Amir as a boy but also foreshadows his inability to defend Hassan from Assef. When Amir becomes an adult, he is still haunted by the memory of not defending Hassan and lives with a guilty conscience. The meaning behind this quote motivates Amir to face his demons and "become good again." In order to find redemption, Amir must literally and figuratively stand up to anything. Baba's quote was an accurate description of how Amir refused to face his demons throughout his life until he decided to seek redemption. 

Baba's description of Amir is fairly direct.  Baba notes that Amir does not stand up for himself.  He lets Hassan do the dirty and challenging work of fighting off the thugs and bullies that intimidate him.  In observing this tendency, Baba notes that Amir does not embody any values that Baba finds desirable, such as being good at sports and demonstrating physical strength.  Amir is a thinker, a poet, a words- person. These are qualities that Baba does not respect and does not "get."  It is in this light where he observes that a child who does not fight back or show defiance when in trouble becomes someone who lacks values as an adult.  In a reminder of story's opening of "to be good again," the father's description of his son is a reminder of the redemption idea that runs through the story.  

The quote is a reflection of how Amir was as a child, something that is later confirmed with how Amir does not stand up for Hassan when he is being raped and violated.  For his part, Amir overhears the quote and is deeply hurt.  Perhaps, this is a reflection of how accurate the quote might be in describing Amir, and becomes the subconscious driving force for him to "become good again.