Lebensraum is the land or territory that a country's leaders believe it requires in order to grow and flourish. The word lebensraum has almost always been used to talk about Germany's strategy in World War II.
Lebensraum was used early in the twentieth century by Germans explaining a plan to colonize other countries, but the word is most strongly associated with Nazi Germany. The Nazis used the idea of Lebensraum, literally "living space" in German, as the basis of their policy for the Germany's expansion. Lebensraum was the excuse for the invasion of Poland and the desire to occupy Russia, the the goal of using resources from those countries to benefit Germans and Germany.