An example would be a deal in which you are the agent of the seller. You have two offers to buy their property and they're almost identical in price and contingencies. Your seller instructs you to accept the offer from Agent B without going back to both buyer agents for further offers because your seller "doesn't like Agent A". Though you may feel that it is unfair to Agent A and their buyer, you must obey the instructions of your seller.
Though confidentiality is discussed separately, it is also a component of the loyalty piece. Disclosing anything about your client without their express consent would not be in keeping with this loyalty requirement.
Beyond that, the fiduciary duty of disclosure would include just about any knowledge the agent had that might benefit their client in the process. If working for the buyer, and you knew the seller was in a financially stressed situation, you would disclose that to help your buyer in the negotiation.
Whether on the buyer or seller side, you are expected to advise on price, inspections, negotiations, repairs and many other facets of the transaction. If it's not something you're expected to know, you are expected to advise your client in how to get the information.
Know Your Fiduciary Duties as an Agent and Perform them Diligently:
Over many years, common and statutory laws have evolved as regards agency and fiduciary responsibilities. As real estate representation evolves, there are new court cases every year in many states that add to the expectations of our clients and courts.
Know the laws in your state that pertain to when you are an "agent" and when you are not. When you are, know what's required of you and carefully and diligently try to perform accordingly. Don't try to offer advice or services for which you are not qualified, but know that there have been court rulings that indicated the real estate professional should have known where to send the client for the information they needed.