You blow a whistle to give a loud warning that everybody can hear. If someone in a game does something wrong, a referee might blow a whistle to stop the game, and penalize the player. Suppose a group within large organization--a company or a government agency--is breaking the law. Within the group, many people know about it. There is enormous pressure to keep it secret. Everyone would be under orders not to talk to people higher in the company, not to talk to law enforcement, not to talk to the newspapers. In order to stop the wrongdoing, an employee would have to be disloyal to their boss. If the boss found out about it, they might get punished in some way or even be fired. The act of reporting wrongdoing, against boss's orders, to someone outside the group is called "whistleblowing." A bold employee might say to the boss "if you don't stop this, I'm going to blow the whistle on it." For example, Edward Snowden was a whistleblower. He said that the US government agency he worked for, the National Security Agency, was going beyond its legal authority. It was spying on US citizens, improperly and secretly collecting phone records. In order to stop it, he leaked classified information to the press. In his case, he (probably) broke the law, in order to stop a government agency from breaking the law. In the United States, some state and federal laws provide some limited protection to some kinds of whistleblowers. If a company is breaking the law, and a whistleblower reports it to the proper authorities, and gets fired, he or she might be able to sue the company in court and win compensation.

Whistle blowing is giving away confidential/secret information about the activities that an organisation is involved in.