Generalplan Ost (translated as General Plan East) was the name of a plan devised by Nazi leaders in 1941 to genocide most of the population in Eastern Europe and resettle it with Germans as well as nationalities the Nazis considered close to theirs, such as Latvians and Estonians. This plan was part of the policy of Lebensraum.
The plan was very detailed, going up to denoting specific percentages of each nationalities to exterminate. For example, the Nazis had planned to execute 80-85% of Poles, but only 50% of Latvians.
Moreover, they ultimately planned to enslave the remaining population left alive, and eugenically turn them into mindless slaves over generations.
The whole plan was never put into practice as the Nazis were never able to annex Eastern Europe and ultimately lost the war. However, that did not prevent them from conducting crimes against humanity and their policy of extermination in the occupied territories, which were also part of the plan in the short-term.