A golem is a mystical being that some exceptionally holy Jewish Rabbis can create from mud and can animate into a semblance of life with special Jewish incantations. There are several legends of golem. The most famous one, and the one relevant to The Amazing Adventures of Kavalier and Clay, is the Prague Golem.
The lengend, though with many versions, says essentially that during the reign of Holy Roman Emperor Rudolph II of Austria, the Emperor began a persecution of Jews during which Jews were either expelled from their homelands or killed outright. Rabbi Loew created a golem in order to protect the Jews of Prague and animated it with the Jewish word meaning "truth" written across its mud forehead.
The golem however became increasingly violent as it grew and turned to killing gentiles as furiously as the gentiles were killing Jews. The Emperor begged the Rabbi to stop the Golem of Prague and in return the Emperor would cease the persecution. The Rabbi agreed and erased the first letter of the word "truth," which then rendered the Jewish word "death" written across the Golem's forehead. It is said that the Golem of Prague still sleeps in the attic of the Old New Synagogue awating a time of further need and that Rabbi Loew also sleeps in the same attic near his Golem. It is this Golem of Prague that Kornblum and Josef discover and smuggle to San Frandisco, CA.