Blackmail means threatening to say something bad about someone unless that person pays some money.
If someone has committed a crime, another person might blackmail them. The blackmailer might, for example, say that if the criminal does not give them a large amount of money, they will tell the police.
A blackmailer may threaten to say something embarrassing that someone unless they hand over a sum of money. For example, it maybe something about an intimacy that they are having that they do not want everyone to know about.
Sometimes, a business may blackmail another business. They might threaten to harm that business in some way unless money is paid.
Blackmail is against the law. People who are caught blackmailing may be sent to jail.