An insider is someone who is privy to the secret information of a company and the secret information could be used to trade.

局内人掌握着公司内幕信息,这些内幕信息可以助你在交易中获利

耶鲁公开课 - 金融市场课程节选

The idea, from the beginning at the SEC, was that insiders should not be allowed to trade on information that is not properly disclosed to the public.

证交会从成立开始就规定,局内人不得利用未公开披露的信息,进行内幕交易

耶鲁公开课 - 金融市场课程节选

If you know the secrets of a company, if you know some good news about the company before the public does, you could buy the shares of the company and experience the profit when the price goes up.

如果你掌握了公司内幕信息,你能比公众更早得知,公司的利好消息,那你就会去购买公司的股票,然后从股票升值中获利

耶鲁公开课 - 金融市场课程节选