Imagine someone working at a candy factory who secretly learns the recipe for a new, super popular candy bar. Insider trading is kind of like that, but with stocks and companies. It's illegal to buy or sell stocks while having secret, important information that isn't available to the public. This information could be things like a company's future earnings or plans for a new product. Insider trading gives someone an unfair advantage in the market, and that's why it's illegal.
