Short answers, pulled from the story.
Lord Curzon, the British Foreign Secretary, proposed the demarcation to define Poland's eastern frontier after World War I. The Commission on Polish Affairs created the suggestion based on spoken language majorities within the former Russian Empire territories.
The March 1921 Treaty of Riga established a frontier well to the east of the Curzon Line. This treaty provided Poland with almost 200,000 square kilometers of land averaging about 480 kilometers east of the proposed boundary.
The Molotov Ribbentrop Pact of August 1939 provided for partitioning Poland along rivers including the San Vistula and Narew without following the Curzon Line. In September after Poland's military defeat the Soviet Union annexed all territories east of the Curzon Line plus Białystok and Eastern Galicia.
Stalin refused to alter the proposed boundary despite Allied pressure because he posed the question Do you want me to tell the Russian people that I am less Russian than Lord Curzon. Western Allies recognized this arrangement in July 1945 making it the permanent eastern border of Poland.
A 1944 article in The Times estimated that between 2.2 and 2.5 million Poles lived east of the Curzon Line in 1931. About 3 million Roman Catholic Poles lived east of the Curzon Line before expulsion operations began.