What was the resistance movement? What did they do?

In the last few pages of the novel, Wiesel leaves out some historical background that would make the narrative clearer. For example, he fails to explain what the camp resistance organization is, and he does not tell us exactly how close to defeat the Germans are. In Chapter Nine, we learn that "the camp resistance organization had decided not to abandon the Jews and was going to prevent their being liquidated." The next day there is a roll call, and the head of Buchenwald announces that the camp is to be liquidated. Ten blocks of deportees would be evacuated each day, and no more food would be distributed.

On April 10, the remaining 20,000 prisoners are to be evacuated and the camp blown up. A siren alert occurs, however, and the evacuation is postponed to the next day. Nobody had eaten anything for six days. The next morning the resistance movement suddenly battles the SS in the assembly place. The SS flees, and resistance takes charge of the camp. At six in the evening, the first American tank arrives at Buchenwald.

From the text, we can infer that the resistance movement was likely a group of political prisoners being held in the camp. For the most part, they successfully avoided detection and sabotaged Nazi activities. In the end, when they were really needed, the group launched an attack that led to the liberation of the remaining prisoners.