According to the story, Pharoah promises to free the the Children of Israel from slavery if God sends ten plagues down onto the Egyptians, as proof of power.
In the final plague, the spirit of God, in the form of an avenging angel would visit each home and kill every first born son. In order for the Israelites to escape this fate, they were advised to brush the blood of a slaughtered lamb on their door frames. This blood would give a sign to the angel that God's chosen people were inside, and so the inhabitants would remain unscathed by the plague. This story is now celebrated during the Jewish festival of Passover.
A juxstaposition of this story can perhaps be seen in Nazi Germany, when the Star of David was painted on many homes and businesses to identify the occupants as Jewish. This time, the sign did not indicate the occupants should be left alone, but instead facilitated boycotts and attacks on the property and people.