A fiduciary is someone legally bound to act in your best interest, even ahead of their own. It is the highest standard of care the law imposes - trustees, personal representatives, agents under a power of attorney, and certain licensed advisors all carry it.
The word matters because much of the financial world does not operate under it. A salesperson can recommend what pays them best as long as it is broadly suitable; a fiduciary must recommend what serves you best, disclose conflicts, and answer for the difference.
A practical test when you sit down with any advisor: ask whether they are acting as a fiduciary for the specific thing they are recommending, and how they are paid for it. A licensed professional can be a fiduciary for one service and a commissioned salesperson for another, and you are entitled to know which hat is on.
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Educational information only, not legal advice. Terms have statutory details beyond these summaries - confirm your situation with a licensed professional.