Brief answer: Persecution of Jews under the Nuremberg Laws, as well as attacks on Jews and imprisoning Jews in concentration camps.
<u>Longer explanation:</u>
Hitler and the Nazis believed in the supremacy of what they referred to as the "Aryan race" -- which was a term they used for the Germanic peoples. They believed their race was superior to "lesser races" like the Jews, blacks and others. Hitler and the Nazis mounted a campaign in Germany to promote their race over others like Jews and Roma (gypsies), etc.
They enacted what are called the Nuremberg Laws, which were passed at a Nazi rally in Nuremberg in 1935. These laws denied citizenship and other rights to Jewish persons.
In November, 1938, there was rampant destruction of Jewish-owned businesses and synagogues and violence against Jewish people. This occurred on the night of November 9 going on into November 10, 1938, and was called "Kristallnacht," or "The Night of Broken Glass." Nazi officials told police and firefighters to do nothing -- to let the violence and destruction occur. In the days after Kristallnacht, the Nazi government said that the Jewish community itself was responsible for all the damage and destruction, and imposed enormous fines against the Jewish community. They also arrested more than 30,000 Jewish men and sent them to concentration camps which were built to incarcerate Jews and any others that the Nazis perceived to be enemies of the German state.
In their campaign for a "master race" as well as in support of their World War effort, the Nazis used Jews for forced labor in concentration camps. They also used Jewish persons and others they deemed undesirable essentially as laboratory rats for doing unethical medical experiments on them. For example, they'd put persons in a pressure chamber to find out how high an altitude they could let their pilots fly before they'd become unconscious from the altitude and pressure. Others of their experiments were even more gruesome.
Ultimately, there was what the Nazis called "The Final Solution" (in the 1940s), which we now refer to as the Holocaust. Millions of Jews, along with other unwanteds, were exterminated in mass killings.
