A more thorough explanation:

Definition: Letters of marque are a license that allows a private citizen to take revenge against citizens or ships of another country. This license is granted by the government.

For example, if a ship from one country attacks a ship from another country, the government of the attacked country may issue letters of marque to private citizens. These citizens can then attack ships from the attacking country to seek revenge.

It's important to note that letters of marque are not commonly used today. The power to grant them is held exclusively by Congress, but they have not been granted since the 19th century.