Long story short, the part of Poland which were annexed to the USSR were the result of the 1939 Molotov-Ribbentrop Pact between Nazi Germany and the Soviet Union, and which the Soviets occupied in 1939. Stalin then compensated Poland with German territories tot he west.

The longer story is that Poland was a country which existed until 1795, when they were partitioned by Russia, Austria and Prussia.

In 1815, in the aftermath of the Napoleonic War, the Congress of Vienna created "congress Poland" a nominally sovereign country under "personal union" with the Russian crown. Congress Poland was dissolved after a rebellion in the 1831.

At the end of WWI, Poland gained independence by the will of the victorious of allie, and subsequently won a war against the newly founded USSR in 1921, which ended in the treaty of Riga.

Because the Interwar Polish border was settled at the Peace of Riga in 1921 following the Polish-Soviet War. What this meant is that Poland actually got quite a bit of what was territory of the Russian Empire (outside of Congress Poland itself) before the war.

Stalin was very interested in recovering the territories of the former Russian Empire, and contributed to his decision to partition Poland with Germany.