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 a sexual relation 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 illegal (against the law).