A key is a device used to open or close a lock, whether mechanical or electronic.
A key is a small piece of shaped metal with incisions cut to fit the wards of a particular lock, which is inserted into a lock and turned to open or close it. Throughout the wasteland, these keys open specific doors or containers, some of which are inaccessible otherwise. There are also keycards, access cards, passcodes that serve as keys.
Such keys are found in the open in the game world or in containers, or on corpses. Passwords are found on notes, on computer terminals, or given in prerecorded messages or on holotapes heard during quests. Keys have no weight.