Operant conditioning, defined and coined by B.F. Skinner (during his famous "Skinner Box" experiments), relies upon reward and punishment. Unlike classical conditioning, where the environmental stimulus comes before the naturally existing reflex (think how Pavlov trained dogs' salivation reflex to trigger after hearing a bell ring), in operant conditioning an environmental stimulus (punishment or reward) is given or withheld after the designated behavior occurs.
Therefore, operant conditioning may be applied in the language acquisition of a toddler. In positive reinforcement, a reward may be given to the toddler when he or she uses a desired word. For example, if teaching a child to ask for milk, the child would be rewarded with a glass of milk only when they say the word "milk." On the other hand, the reward of the glass of milk would be withheld if the toddler says a word other than "milk"; this is negative reinforcement.