The main theme running throughout the novel is good versus evil. Harry Potter represents the good, whereas Voldemort is evil personified.
Harry's upbringing as the oppressed foster child raised by relatives in a very conventionalized English household forms him into an unspoiled and compassionate adolescent who knows what it is to suffer and be treated unfairly. He is very grateful for the friendship, love, and community he enters into in Hogwarts—the simple nurture and caring that he receives in the wizarding world is new to him. He is, as a result, a well-balanced individual who does not have an inflated ego or high self-opinion despite his importance in his new world.
He stands for good because of his attitude of basic goodness, decency, kindness, and gratitude to others. He attracts to him like-minded people who are unpretentious and caring.
Voldemort, filled with anger and bile, is the opposite of everything Harry represents. He and his followers want to oppress others, rob them of their souls, and make them suffer. If Harry is about love and humaneness, Voldemort is about sucking all the joy out of others and dominating them completely. He revels in sadism and cruelty.
In this epic battle between good and evil, the soul of the wizarding world is at stake. If Harry's side doesn't win, everything good about the wizarding world that has been built up over the centuries will be destroyed. The stakes are very high, and Harry seems like an unlikely leader for this battle.
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.