A fiduciary duty is a legal duty that means a person is acting in the best interests of another person. It is a requirement to act in the best interest of the other person. At different times, other people may have control over another person's assets, such as when the person is incapacitated and has a guardian who controls their finances. There is an onus on the guardian to only spend the other person's funds in line with what the person would do and in a beneficial manner for the person and not the guardian.
Another situation where a fiduciary duty arises is within probate law and specifically the onus and legal responsibility of the executor of the estate. Although probate isn't the only time this duty is created. The executor has to act in the best interest of all of the beneficiaries of the will and not just for their own benefit. If a person breaches their fiduciary duty, they could be sued and held financially and criminally liable.