Okay kiddo, imagine you and your friends have a big toy box filled with all your favorite toys. Everyone can play with any toy they want, right? Well, a long time ago, the people who ran the country had a big "toy box" too, but instead of toys, it was full of churches and all their priests.
Some of the people who ran the country thought that the churches and their priests should follow some new rules that everyone else had to follow too. They wanted the priests to be paid by the government and not by the churches anymore. They also wanted the priests to be elected by the people instead of being chosen by the church itself.
This made some of the priests and their followers very mad because they believed that only the church should control the church's rules and laws. They thought that the government should not be telling them how to worship or who to choose as leaders.
Eventually, the government won and they passed a law called the "civil constitution of the clergy". This law made the priests swear loyalty to the government and obey the new rules. Some priests refused to do this and were forced to leave their churches and homes.
In the end, the civil constitution of the clergy caused a big divide between the church and the government and it took a long time for them to work together again. But just like you and your friends in the toy box, sometimes people have to compromise and share their toys to get along.