Short answers, pulled from the story.
Lebensraum is a German word meaning "living space." The term was first used by geographer and biologist Oscar Peschel in his 1860 review of Charles Darwin's On the Origin of Species, where it described the natural habitat of a species. The geographer Friedrich Ratzel later applied it to human populations in his 1897 book Politische Geographie and expanded it in his 1901 essay Lebensraum: A Biogeographical Study.
Adolf Hitler adopted Lebensraum as a core principle in Mein Kampf (1925) and in the unpublished Zweites Buch (1928), arguing that Germany required the conquest of Eastern Europe, particularly Ukraine and Russia, to solve German overpopulation and secure agricultural resources. On the 3rd of February 1933, Hitler told German generals that the conquest of Lebensraum in Eastern Europe and its ruthless Germanisation were the ultimate objectives of Reich foreign policy.
Generalplan Ost was a Nazi blueprint for the conquest and colonisation of Eastern Europe, commissioned by Himmler on the 21st of June 1941 and approved by Hitler in May 1942. It called for the deportation, extermination, or enslavement of Polish, Ukrainian, Russian, Belarusian, Czech, and other Slavic populations, and the resettlement of the vacated territories by over 10 million German colonists.
The Septemberprogramm was a secret German war-aims document authorised in September 1914 by Chancellor Theobald von Bethmann Hollweg. It called for Germany to annex a Polish Border Strip of approximately 30,000 square kilometres and to ethnically cleanse it of Slavic and Jewish populations, replacing them with German colonists. It established Lebensraum in the East as an official German government goal a generation before the Nazi period.
Hitler studied the destruction of Native American peoples during the United States' westward expansion and viewed it as a direct template for German expansion into Eastern Europe. At a 1941 conference he stated that Germans should "treat the original inhabitants like Indians." He also saw the First World War Allied naval blockade, which caused food shortages in Germany, as proof that only territorial expansion could free Germany from dependence on food imports.
Several historians have argued that it did. The ideological imperatives of Lebensraum drove the murder of Soviet prisoners of war, the starvation of occupied populations, and the diversion of resources to genocide, all of which weakened German military capacity. Hitler's refusal to surrender conquered eastern territories for strategic reasons produced military disasters, and the Red Army's victories at Stalingrad (August 1942 - February 1943) and Kursk (July-August 1943) ultimately shattered German offensive power in the East.