A more thorough explanation:

Definition: Restraint of princes is an old term that means an embargo, which is a government's detention of private ships belonging to an offending nation found in the ports of the aggrieved nation. This term is sometimes used in marine-insurance contexts.

Example: During wartime or peacetime, a government may impose a restraint of princes on a nation's private ships found in its ports. For instance, the President called off the embargo of Iraq's ships after the war ended.

Explanation: The example illustrates how a government can use restraint of princes to detain private ships belonging to an offending nation found in its ports. In this case, the President imposed an embargo on Iraq's ships during the war, but lifted it after the war ended.