Okay, kiddo, so you know how sometimes people might trick others by telling lies or making up stories? That's kind of what happened in the case of the Taxil hoax.
A long time ago, there was a man named Leo Taxil who didn't like the Catholic Church. So he decided to make up some stories and pretend they were true to make fun of the Church and the people who followed it.
One of the stories he made up was about a secret group called the Palladium, who supposedly worshipped the devil and did all sorts of bad things. Taxil wrote books and gave speeches about the Palladium, convincing some people that it was a real thing.
But then, later on, Taxil admitted that he had made the whole thing up. He had fooled a lot of people, but it was all just a hoax.
So, basically, the Taxil hoax was when someone pretended something was true, even though it wasn't, just to trick people and make fun of something else.