This section of the novel comes towards the end, when they have let the camp and have been stationed in Gleiwitz for three days. The prisoners have had to run so far to get there through appalling conditions and many have died or have become severely weakened.
On the third day, at dawn, we were driven out of the barrack. We threw blankets over our shoulders, like prayer shawls. We were directed to a gate that divided that camp in two. A group of SS officers stood waiting. A word flew through our ranks: selection!
It is hard to underestimate the terror that this word word have struck into the prisoners. Also note the simile used to describe the blankets that they threw over their shoulders - they are compared to prayer shawls, both ironic considering how the narrator has lost his faith in a God that could permit such things to happen and also poignantly tragic considering what is just about to happen.
Note how the narrator managed to save his father, however:
My father was sent to the left. I ran after him. An SS officer shouted at my back:
I inched my way through the crowd. Several SS men rushed to find me, creating such confusion that a number of people were able to switch over to the right - among them my father and I. Still, there were gunshots and some dead.
Thus the narrator is able to save his father by breaking ranks and causing confusion among the prisoners. This enables his father to re-join the other prisoners selected to live.