Before Conditioning - the unconditional stimulus would be the dog food, and the unconditional response would be the dog getting hungry or excited when seeing the food. However, my dog here is unaware of the food unless there is a way to get his attention.
My dog needs to be fed everyday, but I have a hard time getting him to stay still and come eat.
During conditioning - A new stimulus is introduced. At this stage, the whistle is considered neutral. By itself, it produces no response. However, when used in conjunction with food, it will produce an unconditioned response, in which the dog will salivate in response to the food and hear the whistle. Over time, these things will become correlated.
Every time I need to feed my dog, I will take him to the food and blow the whistle!
After Conditioning - The conditioned response here is the dog salivating and coming to me at the sound of the whistle. The whistle is now a conditioned stimulus. I am now able to get my dog's attention easily.
Great! After classic conditioning, I can make my dog come to me AND salivate.