Bread symbolizes the way life in the camp is reduced to its most basic essentials. Early on in his time in the camp, Eliezer is visited by his relative Stein, also a prisoner, who tells him to
Eat! Anything, anytime. Eat all you can.
The word "bread" is a standard way of referring not simply to bread, but to all food. Stein, who has been in the camp for a longer time, understands that getting enough food is the key to survival in a situation where the prisoners are overworked and badly underfed. Stein is advising Eliezer not to take any bit of food for granted. Bread is a symbol of life itself.
Later, we see how Eliezer's humanity is reduced to the most basic survival instinct, symbolized by his incessant focus on food:
The bread, the soup—those were my entire life. I was nothing but a body.
Just staying alive has become such a daily challenge that all else is jettisoned. Bread has become a currency more precious than gold. In fact, Eliezer is happy when he is saved from losing a tooth with a gold filling because he knows that some day he may be able to trade the filling for a piece of bread.
Bread is in such short supply that any crust is seized. People will trade what is much more valuable in the outside world for a piece of bread so that they can stay alive another day.