A more thorough explanation:

Fidei-commissarius is a legal term that refers to a person who has equitable rights in property and usually receives the rents, issues, and profits from it. Another term for this person is cestui que trust, which means "he for whose benefit the trust was created."

For example, if a person creates a trust and designates their child as the cestui que trust, the child would be the beneficiary of the trust and entitled to receive the benefits of the trust property.

In simpler terms, a fidei-commissarius or cestui que trust is someone who benefits from a trust or has a right to receive income or profits from a property.