Golden Keys are needed to open the Gold Chest aboard Sanctuary.
Each weapon you get from the Golden Chest is guaranteed to be a high rarity tier and could include a Legendary. Some may also contain added cosmetics, but the guns are the real stars here. All characters controlled by that profile will share a common pool of Golden Keys.
Golden Keys are stored at the profile level. When Golden Chests are unlocked, they typically contain rare purple or magenta weapons or rare purple or blue relics.