Pavlov noticed that dogs began salivating at the mere sight of the person who regularly brought food to them. For the dogs, the sight of this person had become a stimulus.

Pavlov noticed that dogs began salivating at the mere sight of the person who regularly brought food to them. For the dogs, the sight of this person had become a conditioned stimulus.

Explanation:

During his studies on dog's digestive reaction Ivan Pavlov initially explored the concept of classical conditioning. He found the dogs were instinctively salivating in reaction to food, but the animals also started drooling if they ever saw the lab assistant's white coat that provided the food. In classical conditioning the conditioned stimulus is a formerly neutral stimulus which gradually tends to cause a conditioned response after being acquainted with the unconditioned stimulus.

For an instance, the scent of food is an unconditional stimulus and its response is a sensation of hunger. Now think  that you too heard the sound of a whistle as you tasted your beloved stuff. Although the whistle is irrelevant to the food's scent, if the whistle's noise was combined with the scent several times, the sound alone would inevitably activate the conditioned response.