"A precious heirloom passed down among the Gerudo. It deflects lightning strikes and is traditionally worn only by the Gerudo chief"
The Thunder Helm is a Key Item in Breath of the Wild. It is an ancient hierloom of the Gerudo people with the ability to protect its wearer from lightning strikes. When Link arrives in Gerudo Town, he learns that the Thunder Helm has been stolen from the chief Riju by the Yiga Clan. Link is required to recover this item from the Yiga Clan Hideout in the Karusa Valley in order to gain Riju's trust. After the helm is returned to Riju, the sight of her wearing it will restore one of Link's memories. Riju wears the helm during the fight against Vah Naboris to protect herself and Link from Naboris's lightning strikes.
After defeating Vah Naboris, Link can examine the helm, which will cause Riju to ask if Link wishes to borrow it. If he claims he does, she will give Link a quest to help all the inhabitants of Gerudo Town. She allows Link to use the Thunder Helm after he completes this quest. The Thunder Helm gives Link the "Lightning Proof" trait if worn.