A Key Encryption Key or KEK is simply a key that is solely used to encrypt keys. The keys that are encrypted are usually keys that have a specific meaning such as domain specific keys. It could also be that the encrypted keys have shorter life time such a as session keys.
A KEK may be used in combination with ECB mode as the encrypted key material should be indistinguishable from random. This is one of the few places where ECB encryption does not leak information. Note that it is an extremely bad idea to encrypt structured information with ECB, including asymmetric keys.
In general ECB is not a good key encryption algorithm. Currently I would advice a scheme such as AES-SIV or AES-GCM-SIV. This will create a randomized ciphertext if the key only differs by a single bit. The calculated synthetic IV in SIV mode double acts as an authentication tag. This means that any changes (deliberate or not) of the key value can be detected. Beware that it is still possible to e.g. swap encrypted key values though - this is something that the wrapping algorithm itself cannot provide protection for.
Key encryption is sometimes also called wrapping.