Okay, so let's imagine you're a little kid who knows about letters and words in English. Well, in China, they don't use letters like we do. Instead, they have something called "characters."
Chinese characters look kind of like little pictures, but they actually represent different sounds and meanings. So, if you put a few characters together, they can make a word.
Now, Empress Wu was a very important woman in ancient China. She lived a long time ago and was very powerful. And, she had her own special set of characters that she used to communicate. These characters were designed just for her, kind of like a special secret code.
The characters of Empress Wu were very ornate and intricate. They had lots of curvy lines and different shapes to make them unique. And, because they were so special, only she and a few other people knew how to use them.
So, just like how you might have a secret language with your friends, Empress Wu had her own secret characters that she used to communicate with those close to her. Cool, huh?