When you say "they," I assume you are talking about the Russian prisoners who are in the camp near to the training camp that Paul goes to after his leave.
These prisoners are being mistreated quite badly by the Germans. They are essentially being given nothing to eat. The conditions in which they live are so bad that very many of them have contracted dysentery.
The only way that these Russian prisoners of war can survive is by going through the garbage from the training camp for the German soldiers. In fact, they are so desperate they eat even things that are not food.