CS stands for Conditioned Stimulus which is a stimulus which after association with an unconditioned stimulus, comes to trigger a conditioned response.
UCS stands for UnConditioned Stimulus that produces the reflexive / natural response.
When the CS is repeatedly presented in the absence of the UCS (food, in this case), the CR will "die out" in a process called extinction. The process extinction includes diminishing of a CR. In classical conditioning extinction happens when a UCS does not follow a CS and in operant conditioning it happens when a response is no longer reinforced.