An internment camp is a prison-like facility where people are held, usually in large groups, without a trial.
During World War II, Nazis sent people, mostly Jews, to internment camps during the Holocaust.
Magneto, a survivor of the camps, worried that people would do the same to mutants.
Fulfilling that fear, when Senator Robert Kelly announced his bid for President of the United States of America he promised he would send mutants to internment camps.
The Nazi's use of internment camps was never specified in any series.