It was commonly used in medieval times, particularly in England, to indicate the lineage of a person.
For example:
* Fitzwilliam means "son of William"
* Fitzgerald means "son of Gerald"
* Fitzroy means "son of the King"
It's important to note that the prefix "Fitz" is often used as a surname, and it's not always clear whether it refers to a specific father or just indicates lineage.