To blackmail someone is to use secret information to get something from them, usually money. Blackmailing is a crime.
Blackmail is a type of threat. For example, if a politician's assistant knew the politician was having an affair, the assistant could blackmail the politician by threatening to tell the press. Blackmailers usually want money in return for keeping something a secret, but an employee with dirt on a boss could blackmail the boss to get a promotion. Anyone making threats and demands in return for keeping a secret is engaged in blackmail.