Adding something the dog will work for, to strengthen (increase the frequency of) a desired behavior. For example, giving a dog a reward* for sitting in order to increase the probability that the dog will sit again.
*Trainers and owners typically give some form of food treat. However, not all dogs are interested or motivated by food and then a toy or other satisfying item or activity is substituted. The reward needs to be something the dog is interested in and likes. Examples, a special toy or object, or praise, or a few seconds of playing tug.
Positive reinforcement is often combined with Clicker Training.
There are no fear or aversive outcomes associated with Clicker Training. It is appropriate for dogs of any age, size, breed or life experiences. Clicker training is a science based method that utilizes the theories of operant and classical conditioning. When properly used, clicker training is a effective, fast means for guiding or connecting with a dogs mind.
Download and watch some how-to do it, Click Training video's. For beginners, start with 'Charging the Clicker'.
Reinforcement is a term used in Operant conditioning and behavior analysis. Reinforcement is used to increase the rate or probability of a behavior (sitting) in the form of a "response" by the delivery or emergence of a stimulus (a treat) immediately or shortly after performing the behavior.
The response strength is assessed by measuring frequency, duration, latency, accuracy, and/or persistence of the response after reinforcement stops. Experimental behavior analysts measured the rate of responses as a primary demonstration of learning and performance in non-humans (example, the number of times a pigeon pecks a key in a 10-minute session).
A reinforcer is the stimulus, event, or situation that is presented or otherwise emerges, when the response behavior is performed. --(source)
More about Clicker Training.