A more thorough explanation:

filus

A filius is a Latin word that means "son" or "descendant". It is used to refer to a male child or a male descendant of a family.

• John is the filius of his parents.
• The filii of the royal family were expected to follow certain traditions.

In the first example, "filus" is used to refer to John as the son of his parents. In the second example, "filii" is used to refer to the male descendants of the royal family.