Most people find it somewhat surprising that The Jungle Book is seven short stories. Most of them are unrelated to each other, and some of them are not even in the jungle. Most people assume that The Jungle Book is the story of Mowgli and his adventures with jaguars, monkeys, snakes, etc. For the purpose of this answer, I'm going to take the book as a whole. I don't believe all readers will agree that there is a single main idea. Different readers are going to find certain themes more central than other themes. For me, there has always been a large focus on the theme of family. Mowgli might not be a wolf, but that doesn't change the sense of family and belonging that he experienced with them for many years. The wolves themselves are representative of a family unit as well. They are a pack animal. Rikki-Tikki feels a sense of belonging with the family that rescued him, and he fights to protect them. That leads to another strong theme present throughout the stories: the theme of courage. Mowgli bravely faces the unknown as well as Shere Khan himself, and Rikki-Tikki places himself in grave danger when he goes down the snake hole.