Answer: In fiction or non-fiction, a foil could be a character who contrasts with another character; most of the time it is the hero, to highlight qualities of the other character. In a few cases, a subplot can be utilized as a foil to the main plot.
Wiesel is referring to the words their new overseer said to them whenever they got to Auschwitz. EH told them that they had avoided the worst part- the selection- and now had to keep faith and that they would survive this. He told them to have camaraderie among each other because it would help them get through this.