A bell ring is a neutral stimulus that, through classical conditioning, becomes associated with the presentation of food and elicits a conditioned response.
A neutral stimulus is something that does not naturally elicit a response before conditioning. For example, the sound of a bell ringing before it is associated with food.
A conditioned response is a learned response that occurs as a result of an association between a previously neutral stimulus and an unconditioned stimulus. In this case, salivating at the sound of the bell ringing after it has been paired with food.
An unconditioned stimulus is something that naturally elicits an automatic response without any prior learning. In this case, the presentation of food which naturally causes salivation.