Stimulus control, as used in the field of behavioral psychology, (hyperlink?) refers to behavior that exists in the presence of a stimulus, but disappears when the stimulus is missing. This type of behavior that promotes certain behaviors is called a discriminative stimulus.
For instance, when you are driving a car and see a red light you put on your brakes. In this case, the red light is a discriminative stimulus that triggers the braking behavior. In this case stepping on the brake is a learned behavior as a response to the discriminative stimulus of the red light.