Juku is like extra school lessons after regular school. Imagine you finish school for the day but instead of going home and playing with toys or watching TV, you go to another class where you learn even more things.
Some kids go to juku because they don't understand everything they learned in regular class, and they want extra help. Other kids go to juku because they want to learn more things and get ahead of their classmates.
Juku is very popular in Japan because some kids want to do well on their exams so they can go to a good high school and then a good university. In Japan, there is a lot of pressure to do well on exams and get good grades.
Juku can be fun too! Some juku classes teach things like art or dance. And you get to meet new friends who also like to learn.