Classical conditioning refers to
closely associating a neutral stimulus with one that evokes a reflexive response so that eventually the neutral stimulus alone will evoke the same response.
Think of the process of training your dog to fetch a thrown ball and return it to you. When you begin the process, you could put the ball (the neutral stimulus) on the ground close to the dog and wait for him/her to notice it. When the dog noses the ball, you would give it a treat as a reward. Repeated often enough, the dog would soon be coming directly to the ball every time s/he saw it.
Next, you would throw the ball some distance away while the dog is sitting next to you. The dog would run to the ball. When the dog picks up the ball in its mouth, a reward would be given by a helper who is close to where you are throwing the ball.
The third step would be for the dog to go to the thrown ball, pick it up, and bring it back to you because you have called the dog to come back to you. When the dog is back at your side and has dropped the ball, it receives its reward (the reflexive stimulus). The dog will quickly learn to repeat the trick and will continue to do so even when the reward is discontinued.