If conditioning has taken place, the neutral stimulus becomes the conditioned stimulus. Developing and strengthening a conditioned response happens through pairing a neutral stimulus with unconditioned stimulus that trigger a conditioned response.
Example of this is a dog that naturally salivated in response to food, but the dog also began to drool whenever they saw the man (which is previously neutral stimulus) who become associated with an unconditioned stimulus (the food) because he delivered the food where it naturally and automatically triggered a response (salivating).