Genesis 19:26 (KJV) 26But his wife looked back from behind him, and she became a pillar of salt. The moral is that Christians shouldn't look back at their sinful ways as that is the path to death. Actually, this is one interpretation. The other is obedience. Being she turned back, she was disobedient. You have to remember, if you picked up a stick on the sabbath, you were struck dead. Obedience is greater than sacrifice. In the movie "Once upon a time in America ", Noodles meant the same, " the path leading to death".

God allowed the honest Lot and his wife to leave Sodom before he destroyed it for the sins of its inhabitants But Lot and his wife were not allowed to look back at the wrath of God Lot's wife disobeyed and glanced back - turning into the pillar at salt at once

Yes it means to be struck dead. The term comes from the Bible.