In operant conditioning, a primary punisher is a reaction that occurs after an undesired behavior that is intended to curtail that behavior. This can take the form of a change in the subject's environment or an unpleasant stimulus. Primary punishers are usually "innate" punishers, or punishers that impact the subjects well-being; hunger, environmental temperature (hot/cold), electric shock, hunger, thirst, etc.
For example, rats can be trained to avoid certain parts of a cage through the use of mild electric shocks. On the other hand, secondary punishers are learned; parking tickets, scolding, error buzzers on game shows, etc.