This moment comes after an intense march to the camp. All the prisoners have been made to run non-stop. None of them know how long they've been moving; they only know if they stop they will be killed. Elie encourages his father to keep running the entire time, risking himself to ensure that his father stays with him.
When they arrive at the camp, they learn that there will be another selection. As they file past the guards, Elie's father is chosen to go to the left. This means he will be sent to the crematorium. Elie is sent to the right, meaning he can go on working a while longer. Yet Elie is not ready to let his father go. Elie runs over to the left line, causing chaos and confusion among the prisoners and guards. Because of the commotion, Elie and his father are able to sneak back over to the "good" line, saving both of them.