Okay kiddo, so a long time ago, there was a place called Spain, and people there had different religions, like Islam and Christianity. But, the rulers of Spain at the time didn't like Islam and wanted everyone to be Christian. So, they made laws saying that Muslim people had to become Christian or they would be punished. This is called a forced conversion, because the Muslim people didn't have a choice.
This was a very scary and unfair thing to do, because people should have the right to choose what religion they want to follow. Many Muslim people had their homes and belongings taken away from them, and some were even put in jail or killed if they didn't become Christian. This made life very difficult and sad for a lot of people.
Thankfully, people realized that this was wrong and things have since changed. Now, in Spain and in many other places around the world, people have the freedom to practice any religion they choose, without fear of being forced to convert.