When private information is used illegally to make money in the stock market, it's called insider trading. If someone who works at an investment bank sells shares based on information that isn't public yet, it's insider trading.
In the world of finance, an insider is anyone with access to information that isn't available to the general public — like CEOs of companies, government officials, and bank executives. Buying and selling stocks, or trading, on the basis of insider knowledge, is illegal. It's not fair for the president of a corporation to tell her friends that an upcoming merger will boost the value of the company's stock when ordinary investors don't have that same information.