Okay kiddo, so a realgymnasium is a school in some places that's kind of like a mix of a regular school and a gymnasium (not the building with weights and equipment, but the type of school).
At a realgymnasium, students usually get a lot of exercise and physical education, like gym class, but they also get a good education in all the other subjects like math, science, history, and language arts.
It's important for kids to learn both how to take care of their bodies and how to learn and grow in their minds, and that's what a realgymnasium tries to do.
Some people really like going to realgymnasiums because they get to stay active and healthy while also learning a lot of interesting things.