This is a direct quote from the book at comes from a conversation between Baba and Rahim Khan about Amir. Baba talks about the way that Amir will not fight back, and how Hassan has to stop him getting hurt by others. He says this quote to describe how it is so vital to be able to stand up for yourself in this world, because if you can't even do that, you won't be able to stand up for anyting. For a man such as Baba, who is defined to such a great extent by his principles, having a son who is not able to stand up to injustice, or to those who seek to oppress others, is a terrible thought. For the majority of the text, Amir does show himself to be just that kind of individual: at every turn he shows himself unable to stand up for either himself or for principles or people who are important to him. Most importantly, he fails to act during Hassan's rape, but also note how he does not act to protect the woman from being raped during their flight from Afghanistan and he in fact does what he can to dissuade his father from acting. It is only when he goes to visit the dying Rahim Khan in Pakistan that he is shamed into action. Note what Rahim Khan says to him:
You know," Rahim Khan said, "one time, when you weren't around, your father and I were talking. And you know how he always worried about you in those days. I remember he said to me, 'Rahim, a boy who won't stand up for himself becomes a man who can't stand up to anything.' I wonder, is that what you've become?"
When Amir learns the truth about Hassan's identity and understands precisely how terrible the betrayal that he committed was, he realises that he has to become a man who stands up for himself so that he can stand up for others. In the end, through his fight with Assef and his seeking of Hassan's son to make ammends for his past error, Amir shows that he has managed to become a man that his father, were he still alive, would be proud of.