What is Gyges' Ring?

Gyges' ring is a hypothetical magical ring which Glaucon and Adeimantus mention in an attempt to show that people only act justly in order to avoid punishment. Gyges' ring is supposed to make its wearer invisible at will and Gyges, the owner of the ring, used it to steal and commit adultery. The two brothers suppose that anyone with such a ring would do so and, therefore, only refrain from injustice out of fear.