Explain the concept of Lebensraum.

Lebensraum was the idea, popularized by Adolf Hitler and the Nazi Party in the 1930s, that the German people should (and would) conquer lands to the east, in particular in Russia. These lands, which would be "cleansed" of Slavic peoples who the Nazis deemed inferior, would then become Lebensraum, or "living space" for the German people. This concept, sometimes compared to the ideology of manifest destiny in the United States, was based on the belief in the racial superiority of the "Aryan" German people. The people who happened to live on these vast expanses of land would either be driven off, killed, or enslaved. While the idea of Lebensraum was not Hitler's originally, he made it a key aspect of his virulently racist party ideology during his rise to power. In his manifesto Mein Kampf, written in prison in 1923 and published two years later, he made the argument explicitly. Claiming that the German people were running out of resources, he pointed to the east as a site for the fulfillment of German destiny:

We must eliminate the disproportion between our population and our area…… Some of this land can be obtained from Russia….. We must secure for the German people the land and soil to which they are entitled.

Lebensraum was the justification for Hitler's aggressions in the late 1930s, including the annexation of Austria and the Sudetenland and his invasions of Czechoslovakia and Poland. Once World War II broke out, Hitler eventually sought to bring the ideology to fruition by invading the Soviet Union. His forces murdered millions of Jews, Slavic peoples, and others with the goal of creating Lebensraum. So this concept, rooted in warped racial theory, is one of the most destructive ideologies in human history.
http://www.bbc.co.uk/history/worldwars/wwtwo/hitler_lebensraum_01.shtml

https://www.historylearningsite.co.uk/modern-world-history-1918-to-1980/weimar-germany/mein-kampf/