Okay, so a long time ago, in a country called China, there was a period of ten years from 1927 to 1937 when a government was formed in a city called Nanjing. This government was led by a man named Chiang Kai-shek, who wanted to modernize and make China stronger.
During this period, some good things happened, like the government building new schools and hospitals to help the people, and setting up modern factories to make new things. But there were also some bad things, like the government being very strict and sometimes using violence to control people who disagreed with them.
In 1937, this period ended when Japan invaded China and a new war started. So in summary, the Nanjing Decade was a time when a new government was formed in China to modernize the country, but it had some good and bad aspects, and it ended when a new war started.