A more thorough explanation:

Term: ad fidem
Definition: Ad fidem is a Latin term used in law to mean "in allegiance" or "owing allegiance". It can be used in phrases like "ad fidem regis" which means "under the king's allegiance" or "natus ad fidem regis" which means "born in allegiance to the king".
Example: John was born in England and is therefore natus ad fidem regis, meaning he owes allegiance to the king of England.
Explanation: The example illustrates the use of the term "natus ad fidem regis" to describe John's allegiance to the king of England. It means that John was born in England and therefore owes allegiance to the king.