The Soviet Union had already occupied Poland by the time of the conference, and the Western Allies were concerned that the Soviets would install a communist government in Poland. They also wanted to ensure that Poland would be allowed to hold free and democratic elections.
The Soviets agreed to allow for free elections in Poland, but they also insisted on keeping control of the country's eastern regions. This resulted in the creation of a new Polish government that was heavily influenced by the Soviet Union.
The Yalta conference was a significant turning point in the Second World War. It helped to shape the postwar order in Eastern Europe, and it laid the foundation for the Cold War.