A more thorough explanation:

Jus gentium is a Latin term that means "law of nations". It has two meanings:

1. It refers to international law, which is the set of rules that govern the relations between nations.
2. It also refers to a body of law in ancient Rome that was applied in dealing with the relations between Roman citizens and foreigners.

The early Roman law, called jus civile, only applied to Roman citizens. It was formalistic and not suitable for an expanding society. The jus gentium provided simplified rules to govern the relations between foreigners and citizens.

For example, if a Roman citizen wanted to do business with a foreigner, they would use the rules of jus gentium to make a contract. These rules were simpler and more flexible than the rules of jus civile.

Over time, the rules of jus gentium became more important than the rules of jus civile. They became the common law of the Roman Empire and were used in all legal cases involving foreigners.