As a part of operant conditioning positive punishment refers to the practice of providing an aversive stimulus following an undesired behavior. This would be like training a lab rat to avoid one corner of its cage by giving it a brief electric shock when it enters that part of the cage. It doesn't take too many shocks to convince the rat to stay away from that corner of the cage.
Remember, in psychological research the terms "positive" and "negative" are used somewhat differently that they are in everyday speech. "Positive" refers to a reaction given in response to a stimulus, while "negative" refers to a lack of reaction to stimulus. Therefore a "positive" punishment refers to an active response (like an electrical shock) whereas a "negative" punishment would be withholding something from the organism (like not presenting food).