A more thorough explanation:

Definition: Opinio juris is a Latin phrase that means "an opinion of law or necessity." In customary international law, it is the second element necessary to establish a legally binding custom. Opinio juris denotes a subjective obligation, a sense on behalf of a state that it is bound to the law in question.

Overview: Opinio juris is an unsettled and debated notion in international law. The International Court of Justice reflects this standard in ICJ Statute, Article 38(1)(b) by reflecting that the custom to be applied must be "accepted as law".

Example: For example, if a state consistently follows a certain practice, such as allowing foreign ships to pass through its territorial waters without permission, and other states also follow this practice, then it may become a customary international law. However, for this practice to become legally binding, there must be an opinio juris, a sense that the state is bound to this practice as a matter of law.

Explanation: The example illustrates that opinio juris is necessary for a customary international law to be legally binding. In this case, if the state allows foreign ships to pass through its territorial waters without permission, and other states also follow this practice, it may become a customary international law. However, for this practice to be legally binding, there must be an opinio juris, a sense that the state is bound to this practice as a matter of law.