Insider trading is illegal use of information of a company for one’s personal profit. This term is commonly used to refer to buying or selling of stocks on the basis of information that is not available to the public. Let’s consider an example of a manager who has exact information about the stocks which is not disclosed and he uses that information to buy stocks of his own company knowing the profit he will manifest out of this action, then it is called insider trading.
Other example of it could be an employee trading the information about his company’s future planning regarding a product with some other competitor company in the market.