the answer is: The neutral stimulus causes no response.
Without being paired by any situation, neutral stimulus would not mean anything to the subjects. Because of this , the subjects would normally just ignore the neutral stimulus every time they face it.
But, when the neutral stimulus is paired with a certain situation (such as ringing a bell before giving foods to dogs), the dogs will associate the neutral stimulus with the positive reinforcement. To the point where the neutral stimulus transformed into a conditioned stimulus, which make the dogs'mouth to start watering every time they hear the bell.