A “master key” is designed to open a specific set of several locks.
Although these locks have individual keys, they also have a second tumbler system within the mechanism that enables the master key to open any one within that set.
A “skeleton key” is a key ground down by criminals to pick a lock by avoiding the wards in a warded lock.
Warded locks are those opened with a long-stem cylindrical shaft with a single flat rectangular tooth and are mostly out of fashion.
They were called skeleton keys because they had been filed down to the “bare bones.”
An underworld slang term for a skeleton key is screw, which is why convicts call guards in a prison “screws”, because they carry the keys.