This is a good question. Here is a little context. When Elie, his father, and the other prisoners came into the barrack of the camp that they were staying, a gypsy was in charge. He gave permission for the people to sit. When Elie's father asked to use the bathroom, the gypsy struck him out of nowhere.
When the gypsy struck Elie's father, Elie did nothing. He just looked on, and let it happen. The reason why Elie did nothing was because he was shocked. He did not know what to do. Also what could he do?
Elie's father knowing that Elie was in a state of shock and guilt (for doing nothing), whispered to Elie that he was fine and the that slap in the face did not hurt.
Here is the quote from the work, to give you the drama unfold. The Gypsy stared at him for a long time, from head to toe. As if he wished to ascertain that the person addressing him was actually a creature of flesh and bone, a human being with a body and a belly. Then, as if waking from a deep sleep, he slapped my father with such force that he fell down and then crawled back to his place on all fours.
I stood petrified. What had happened to me? My father had just been struck, in front of me, and I had not even blinked. I had watched and kept silent. Only yesterday, I would have dug my nails into this criminal's flesh. Had I changed that much? So fast? Remorse began to gnaw at me. All I could think was: I shall never forgive them for this. My father must have guessed my thoughts, because he whispered in my ear:
"It doesn't hurt." His cheek still bore the red mark of the hand.