Pierre Janet was a man who lived a long time ago and he was very interested in how people's minds work. You know how when you get scared or sad or angry, you start to feel things in your body, like your heart racing or your stomach feeling weird? Well, Pierre Janet wanted to understand why that happened and how he could help people who were feeling those things.
He found out that sometimes people have things happen to them that they can't remember or that they try to forget because they are too scary or sad. These things might have happened a long time ago, but they can still make people feel scared or sad or angry without them realizing it.
Pierre Janet tried to help people remember these things so that they could understand why they were feeling the way they were. He talked to them and asked them questions and sometimes he even hypnotized them to help them remember. He called this "the subconscious mind" and believed that it was very important to understand if you wanted to help people feel better.
Pierre Janet wrote a lot of books and talked to a lot of people about his ideas. Some people thought he was a little strange, but others thought he was very smart and helpful. Even today, people still talk about his ideas and try to use them to help people who are feeling sad or scared or angry.