Thin and wispy flames wreathe your body for the duration, shedding bright light in a 10-foot radius and dim light for an additional 10 feet, You can end the spell early by using an action to dismiss it.
The flames provide you with a warm shield or a chill shield, as you choose. The warm shield grants you resistance to cold damage, and the chill shield grants you resistance to fire damage.
In addition, whenever a creature within 5 feet of you hits you with a melee attack, the shield erupts with flame. The attacker takes 2d8 fire damage from a warm shield, or 2d8 cold damage from a cold shield.
You can find more information on this spell in: PHB.241