Eugenics in Japan is a big word that means some people in Japan believed that some qualities like intelligence and beauty are important and wanted to make sure only people with those qualities can have babies. It started back in the early 1900s when Japan was trying to become a more powerful country. They thought that having smarter and healthier people would help them achieve that goal.
To do that, they created laws that gave the government the power to force people who were considered "unworthy" to have children to be sterilized. This means they couldn't have babies anymore. Some people were considered "unworthy" because they had disabilities, were poor, or were from a different race.
The government kept doing this for many years, even after World War II was over. However, as time passed, people started to understand that it was unfair to control who can have babies and who cannot. In 1996, the Japanese government finally banned eugenic laws.
So, In summary, Eugenics is the belief that people should be born with certain qualities, and in Japan, the government used to decide who should have babies based on those qualities, but it was later banned because it was unfair.