Okay kiddo, the Nero Decree is when a man named Adolf Hitler told a man named Albert Speer to destroy everything in Germany that the Allied armies might be able to use during World War II. This decree was named after a man named Nero who burnt down Rome long ago.
Hitler didn't want the Allies to find anything useful in Germany like factories or bridges or roads that the Allies could use to help them win the war. So he ordered that everything be destroyed, even if it meant leaving little for his own people. This showed how desperate Hitler was to win the war.
But, luckily, not everything was destroyed, and after the Allies won the war, they helped Germany rebuild. So ultimately, the Nero Decree did not have the desired effect and only brought chaos and destruction along.