Stimulus Discrimination is when we learn to respond only to the original stimulus, and not to other similar stimuli. The concept of Stimulus Discrimination follows from the idea of Stimulus Generalization, which is when we respond not only to the original stimulus, but also to other similar stimuli.
For example, whenever you come home from work, the first thing you do is feed your dog. As a result, your dog gets excited as soon as he hears your car pulling up at the driveway, barking and running to the door. Eventually, he begins to get excited as soon as any family member arrives in their car, thinking that he will get fed as well. Everytime he hears any car pull up at the driveway, he starts barking and running to the door. That is Stimulus Generalization. But if none of the other family members ever feed the dog as soon as they arrive home, your dog eventually learns that it is only the sound of your car pulling up at the driveway that's worth getting excited about. That is Stimulus Discrimination, because he learns to distinguish only the specific sound that means food is coming, and learns to ignore all other car sounds as not relevant to his getting fed.