A more thorough explanation:

Definition: A crime that violates international law and is punishable under internationally prescribed criminal law or defined by an international convention. It is an act that is internationally agreed to be of a criminal nature, such as genocide, piracy, or engaging in the slave trade.

Examples:

• Genocide: the deliberate killing of a large group of people, especially those of a particular ethnic group or nation.
• Piracy: the act of attacking and robbing ships at sea.
• Engaging in the slave trade: the act of buying and selling human beings as property.

These examples illustrate how certain acts are considered crimes against the law of nations because they violate basic human rights and are universally condemned. They are not only illegal under national laws but also under international law, and those who commit them can be prosecuted by international courts.