When the conditioned stimulus and the unconditioned stimulus are brought together over and over and over again, pretty soon the conditioned stimulus gives rise to the response.
So when Pavlov, for instance--Well, when Pavlov, for instance, started before conditioning there was simply an unconditioned stimulus, the food in the mouth, and an unconditioned response, saliva.
So, if somebody pokes you with a stick and you say, " Ouch, " because it hurts, the poking and the " ouch" is an unconditioned stimulus causing an unconditioned response.