In classical conditioning, the conditioned stimulus is a previously neutral stimulus that, after becoming associated with the unconditioned stimulus, eventually comes to trigger a conditioned response.
Ivan Pavlov first discovered the process of classical conditioning in his experiments on the digestive response of dogs. He noticed that the dogs naturally salivated in response to food, but that the animals also began to drool whenever they saw the white coat of the lab assistant who delivered the food.
The previously neutral stimulus (the lab assistant) had become associated with an unconditioned stimulus (the food) that naturally and automatically triggered a response (salivating). After the neutral stimulus had become associated with the unconditioned stimulus, it became a conditioned stimulus capable of triggering the conditioned response all on its own.
For example, suppose that the smell of food is an unconditioned stimulus and a feeling of hunger is the unconditioned response. Now, imagine that when you smelled your favorite food, you also heard the sound of a whistle. While the whistle is unrelated to the smell of the food, if the sound of the whistle was paired multiple times with the smell, the sound alone would eventually trigger the conditioned response. In this case, the sound of the whistle is the conditioned stimulus.
The example above is very similar to the original experiment Pavlov performed.
The dogs in his experiment would salivate in response to food, but after repeatedly pairing the presentation of food with the sound of a bell, the dogs would begin to salivate to the sound alone. In this example, the sound of the bell was the conditioned stimulus.