A fiduciary is a person who holds assets in trust for someone else. That person has a fiduciary duty to take care of the money.
Fiduciary comes from the Latin word fidere, "to trust." That's because a fiduciary is the person you trust to hold and watch over your assets until it's time for them to go to another designated person. For example, when a child's parents die, someone is chosen to look after the money left in the will until the child is old enough to manage it himself. Fiduciary can either be a noun to refer to the caretaker or an adjective to describe the trust.