Generalplan Ost (Generalplan for the East) was Germany's plan for dealing with the large Slavic population in Eastern Europe (primarily in the territories conquered from the Soviet Union) that the Germans expected they would be in control of after Operation Barbarossa.

The plan was commissioned by Heinrich Himmler in June of 1941, finished in July 1941 (by the SS office for the strengthening of the German race), and finally approved in May of 1942. The plan was to systemically remove or kill around 30 million Slavs from territories in the Soviet Union. Officially the plan called for a certain percentage of each group to be removed; officially it went:

80% of the Polish population

65% of the Ukrainian population

75% of the Belorussian population

The plan was to starve/work them to death, let disease run rampant among the communities, and expel the various populations further east into Siberia. It was hoped that this combination of methods would lead to the required amounts of deaths.

The idea was to make land for German settlers and allow the territory to be easily "Germanized." There was also an economic incentive, as Germany was facing a food crisis and by allowing their soldiers to just pillage and live off the land, Germany could save more food for their own population while starting the starvation process in Eastern Europe.

Hope this answers all your questions feel free to ask any more.