An object that was seemingly constructed by the Moon Tribe, the Fire Tablet gives its user complete protection from fire. Specifically, with this item equipped, Amaterasu can swim through lava, and she will not take the elemental damage of fire attacks (e.g Orochi and True Orochi's Fire head attacks, Crimson Helm's fire shockwaves, etc.).
After Amaterasu has helped Kaguya with unearthing her rocket at Sasa Sanctuary, she thanked the goddess by giving her the Fire Tablet before departing. Later, the Fire Tablet is used to cross the lava pools of Himiko's palace and the large moat of lava surrounding Ninetails' fortress on Oni Island.
Spoilers end here.
The Fire Tablet seems to be constructed by the Moon Tribe, given the arcane markings on it and Kaguya having it with her before departing for the Moon.