What is the central idea of introduction to the holocaust?

Adolf Hitler's nazis came into power around 1921. To them Aryans were superior to others.

To them, an Aryan was anyone who was European and not Jewish, Romany or Slavic.

And hence this led to anti-Semitic laws against jeas. He and his fellow Ministers. believed Jews were a problem and needed to be gotten rid of. This is what we called ethnic cleansing. Germany invaded it's neighbouring countries soon after these lass took root and many Jews were sent to concentration camps. This is the central idea to your question.