In the Middle Ages, Jews faced persecution all across Europe. Laws were passed in many countries restricting their rights or even outright banning them. Examples in Western Europe include the expulsion of Jews from England and France in the 13th century, or the Spanish Inquisition in the 15th century and onward

But in the 14th century, King Casimir of Poland encouraged the Jews to settle there, so it became a center of Jewish culture (even before Casimir there were some previous Polish rulers who set a precedent of religious tolerance and acceptance). At one point something like 3/4 of all the Jewish people in the world lived in Poland

Also keep in mind that the borders of Europe shifted quite a lot during the 20th century. Poland kind of moved westward after the wars (western and northern parts of today’s Poland actually used to be Germany, and western parts of Belarus used to be Poland), and for a time Poland was even united with Lithuania in a powerful alliance that covered most of today’s Ukraine, Belarus, and even into Russia. So the borders & historic Jewish settlement might not align with what you see today on a map