Insider trading refers to the practice of buying or selling securities based on non-public information that is not available to the general public. This can include information about a company's financial performance, upcoming mergers or acquisitions, or other confidential information. Insider trading is illegal because it is considered to be an unfair advantage that violates the principle of a level playing field in the stock market. It can also undermine public confidence in the integrity of the market and harm the interests of other investors.