Kresy (literally translated as Borderlands) is the part of the Second Republic, which today lie in western Ukraine, western Belarus, and part of Lithuania. These were the parts of Poland lost following on from decisions made at the Yalta and Potsdam conferences in 1944 and 1945. Major cities lost to the Polish state included Lwow (Lviv), Wilno (Vilnius), Brzesc (Brest), Stanislawow (Stanislav), and Grodno (Hrodna).
Most of the Polish population of the Kresy moved to the former German areas given to Poland in exchange for the Kresy areas annexed by the Soviet Union. Until fairly recently many of these Poles dreamt of one day moving back.
Related content on Polish Poland: The Oder-Neisse border (dividing Poland from Germany).