A more thorough explanation:

A letter of marque is a license that gives permission to a private citizen to attack and capture ships or property belonging to another country. This license is granted by the government and is used during times of war.

For example, during the American Revolution, the Continental Congress issued letters of marque to American privateers. These privateers were authorized to attack British ships and capture their cargo. This was a way for the Americans to weaken the British economy and gain an advantage in the war.

Another example is during the War of 1812, the United States issued letters of marque to American sailors. These sailors were authorized to attack British ships and capture their cargo. This helped the United States to disrupt British trade and weaken their military.

In summary, a letter of marque is a license that allows a private citizen to engage in acts of war against another country. It is a way for a government to use private citizens to help in a war effort.