Pavlov's concept refers to the theory of classical conditioning, which explains how a neutral stimulus becomes associated with a reflex response through repeated pairings.
The unconditioned stimulus is a stimulus that automatically triggers an unlearned response. For example, in Pavlov's experiment, the food was the unconditioned stimulus.
The conditioned stimulus is a previously neutral stimulus that, after being paired with an unconditioned stimulus, elicits a learned response. In Pavlov's experiment, the bell became the conditioned stimulus after it was repeatedly paired with food.
The conditioned response is the learned response that occurs when an organism reacts to a conditioned stimulus. In Pavlov's experiment, salivating at the sound of the bell became the conditioned response.