Terasawa Junsei is a person's name, like your name! He is from Japan, which is a country on the other side of the world. People from Japan have different customs and traditions than you do, just like how your family might have different traditions than your friend's family.
Terasawa Junsei is also known for doing something special called ikebana. That's a fancy word for flower arrangement! Sometimes people use flowers to make things look pretty, like putting them in a vase on a table in their house. But in ikebana, people use flowers to make an art piece. They look at the shape and colors of the flowers and arrange them in a way that looks beautiful.
Terasawa Junsei is a very good ikebana artist, which means he can make really beautiful flower arrangements. And lots of people like to look at his arrangements and learn from him. It's like if you're really good at drawing and lots of people like to look at your pictures and learn how to draw like you!