Keys are pieces of ancient technology that allow The Drifter to access areas locked behind doors.
If you have an insufficient amount of keys for a door, the remaining keys will show up red. They are only found on Shadow Corpses, which are behind secret passageways and challenges throughout the World. There are a total of 16 keys scattered around the world. 4 of the 16 keys can be found in each area; Mountains, Lake, Crystal Forest, and Barren Hills.