A Fiduciary is one who practices under an oath to provide you with advice or a level of care that is solely in your best interest and ideally free from conflicts of interest, much like your doctor, lawyer or CPA.
Your typical stockbroker, insurance broker, annuity sales person, or investment consultant at a bank works under the lesser legal standard of "suitability." This means they can recommend investment products that are suitable for your situation, but not necessarily the absolute best option for your financial situation. Under this lower legal standard, they are able to recommend financial products that are in their best interest and not your best interest. In fact, they are required by federal law to act in the best interest of their employer, not in the best interest of their clients. They do not have to disclose their compensation, commissions from financial product sales or conflicts of interest.
In comparison, a Registered Investment Advisor is held to a fiduciary standard, which is considered the highest standard of care available for an investor. Investment Advisors operating as fiduciaries provide all recommendations in writing, all fees or compensation are disclosed and any recommendation provided to you must be absolutely in your best interest.