Ok kiddo, so you know how sometimes people use fancy words to talk about certain times in history? Well, "fin de siècle" is one of those fancy words. It comes from French and it means "end of the century".
Now, when people say "fin de siècle", they are usually talking about a special time in Europe at the end of the 19th century, which was around the year 1900. During this time, lots of things were changing in Europe.
For example, there were new inventions and technologies that were changing the way people lived. People were also questioning old ways of doing things and started to explore new ideas. This led to some people feeling excited about the future and all its possibilities, while others were worried that these changes might lead to problems.
So, "fin de siècle" is a fancy term that refers to this time of change and uncertainty at the end of the 19th century in Europe.