Ancien régime was a time a long time ago in France when kings and queens were in charge. It was a time when there were very strict rules about who had power and who didn't.
The king or queen had the most power and could do whatever they wanted. They had special people that worked for them called nobles who were also powerful, but not as much as the king or queen.
Most of the people who lived in France at that time were called peasants. They were not powerful at all and had to do what the king or queen or the nobles told them to do. They had to work very hard to grow food and make things to sell.
The ancien régime lasted for a very long time until the people who were not powerful got very angry. They wanted to have more say in how things were done and they wanted to be able to have more freedom. This led to a revolution where the king and queen were removed from power and a new system was put in place.