General principles are a source of international law. See Art. 38, ¶ 1 (c) Statute of the International Court of Justice.
"The basic notion is that a general principle of international law is some proposition of law so fundamental that it will be found in virtually every legal system. When treaties and customary international law fail to offer a needed international rule, a search may be launched in comparative law to discover if national legal systems use a common legal principle. If such a common legal principle is found, then it is presumed that a comparable principle should be attributed to fill the gap in international law."
Mark W. Janis, An Introduction to international Law 55 (4th ed. 2003).
Examples of general principles of law: laches, good faith, res judicata, and the impartiality of judges. International tribunals use these principles when they cannot find authority in other sources.