Pavlov's dogs did not need to learn about food. These are stimuli that have been associated with a primary stimulus. In Pavlov's experiment, the sound of the bell meant nothing to the dogs at first. After its sound was associated with the presentation of food, it became a conditioned stimulus. If a warning buzzer is associated with the shock, the animals will learn to fear it.
Secondary stimuli are things that the trainee has to learn to like or dislike.
Examples include school grades and money. A coin or piece of paper money has no meaning to a person who doesn't use that sort of system. Yet people have been known to work hard to gain this secondary reinforcer. In other words, it's hard to toss a fish to a dolphin while it's in the middle of a jump or finding a piece of equipment on the ocean floor a hundred meters below. Marine mammal trainers use a whistle.