Significance of Sodom and Gemorrah

Sodom and Gemorrah are biblical cities which God destroyed because there were wicked people living there. The family of Lot was spared, but told not to look back as they fled. Lot's wife looked back and she was turned into a pillar of salt. The narrator explains that people are not supposed to look back anymore, and that the book he tried to write about his experiences in the war was not good, because it was written by a pillar of salt.