The addition of something desirable to increase the likelihood of a behavior happening in the future.
Reinforcement promotes behavior, it makes it more likely to happen. The “Positive” part of Positive Reinforcement means you add something of value away to reinforce behavior.
In disc dog freestyle Positive Reinforcement is adding the opportunity to get a disc or to do more work. Ramping up the game, throwing a Roller for compliance on the cued Drop, or allowing Next to happen as a consequence of good behavior is Positive Reinforcement.
Positive Reinforcement reinforces behavior by adding something of value when the behavior is performed.