In chapter 9 of "Night," what was the resistance movement and what did they do?

While Elie was in Buchenwald, he learned that there was a resistance movement. There were a number of Jews who were able to pass a non-Jews. They organized and were waiting for the right movement to attack. 

Elie recounts that there was a announcement that all the prisoners should go to the Appelplatz. This was where Elie thought that Hitler would keep his promise of killing all the prisoners. As he was going, he met some people from the resistance who told him to go back.

We returned to the block. On our way there, we learned that the underground resistance of the camp had made the decision not to abandon the Jews and to prevent their liquidation.

As it was getting late and the confusion was great—countless Jews had been passing as non-Jews—the Lagerälteste had decided that a general roll call would take place the next day. Everybody would have to be present.

After a while, Elie says that the resistance decide that it was time to attack. When the SS soliders came in, presumably to kill everyone, the resistance came out with guns and grenades. Eventually the resistance movement was able to push out the German troops. This was when an American tank came into the camp. Finally, freedom arrived for Elie and his fellow Jews.