Narya is described as having the power to inspire others to resist tyranny, domination and despair, as well as having the power (in common with the other Three Rings) to hide the wielder from remote observation (except by the wielder of the One) and giving resistance to the weariness of time. It is also thought to have magical properties.
Círdan, Lord of the Havens of Mithlond, knowing Gandalf's true nature and duty, gave him Narya to aid him in his labours.
Check out the other 2 rings of power Nenya & Vilya