Okay kiddo, let me explain to you what "fiscus iudaicus" means. First, "fiscus" is a Latin word that means treasury, and "iudaicus" means Jewish.
So, a long time ago, the Romans were in charge of a place called Judea (which is now part of Israel). The Romans didn't like the Jewish people, so they made a law that said any Jewish person who paid a special tax would be allowed to practice their religion.
This special tax was called the "fiscus iudaicus." It was a tax that only Jewish people had to pay, and it was used to fund the Roman treasury. The Romans thought that if they made the Jews pay extra money, they could control them and make them behave.
But the Jewish people didn't like this tax. They thought it was unfair and that it was just another way for the Romans to oppress them. This tax was in effect for many years, until the Romans eventually left Judea.
So, that's the story of the fiscus iudaicus. It was a tax that only Jewish people had to pay, and it was used to fund the Roman treasury. The Jewish people didn't like it, but the Romans thought it was necessary to control them.