What is the main idea of Harry Potter?
The main idea of the Harry Potter novels by J. K. Rowling is a sort of adolescent fantasy/wish fulfillment. Basically, most children are treated as somewhat special by their families, but as they reach adolescence, begin to come into contact with the world of adult reality in which most people discover that they are not particularly extraordinary or special, but rather no more important than anyone else.
What the Harry Potter character does is start with a boy who is marginalized, not overly talented at school or sports, and a bit of an outsider, and have it revealed that there is an entire other universe, the world of wizards, in which he is the most important person in the world. Although Harry engages in struggles in the world of wizards, the key point that he is special and matters in some unique way is one common in young adult literature, and serves as a fantasy to counteract the reality that rather than just being a unique member of a small family unit, we all need to adjust to being merely one of over six billion people in the world.