Social Darwinism is the idea that natural selection can be applied to social, political, and economical issues. Natural selection states that those who adapt and evolve to their living conditions. Ex: A Social Darwinist believes that the government should not help the poor to escape poverty because those who can overcome or adapt to their circumstances are superior.
This school of thought was often used to justify one race being superior to another. Nazis believed that due to the harsh environment that the Nordic/Aryan race had encountered during the Ice Ages in north-central Europe, that the weakest had perished and only the strongest of the race had survived to carry on into next generations. Because of this Nazis genuinely believed that the Aryan race was superior to all others and that they needed to spread and overtake the world to spread and create more of the "perfect human beings".