I assume that the "this" to which you are referring is when Baba questions Hassan about Amir's stolen birthday gifts that are found under Hassan's mattress. When Baba asks Hassan if he had stolen the money and the watch, Hassan surprises Amir when he answers with a single word.
Amir understood Hassan's answer as a "final sacrifice for me." Hassan never lied, and he knew Baba would believe him. As Amir came to realize, Hassan must have also recognized that Amir had planted the presents, and he decided to take the blame upon himself rather than tell the truth and have Baba realize that Amir had committed the deceitful act himself.