Do you remember way back in 3rd grade when if you answered a question right in class, or helped a classmate out voluntarily, or picked up a piece of garbage in the hallway and threw it out your teacher gave you piece of candy as a reward? Yeah those were the days. Well, this is an perfect example of a positive reinforcement. A positive reinforcement is a reward given to increase the likelihood of good behavior in a given situation.
This doesn't just apply to 3rd graders it applies to everyone. I think this is important, especially at a young age, because it teaches our brains to associate doing something good for our surrounding community or ourselves with a good feeling or sense of well-being after we do it. I feel positive reinforcement is one key feature in a child's development in order for them to grow up understanding the whole concept of "work hard, play hard". If you work hard and apply yourself, you will be able to someday live your dream. In this case, the 'working hard and applying yourself' is the positive reinforcement for being able to someday live your dream.
Here is an excellent/funny example of positive reinforcement from the T.V show The Big Bang Theory
This not to be confused with negative reinforcement, which is the removal of something that increases the likelihood of good behavior. Like a police officer removing a taser from a caught suspect after a wild police pursuit. The removal of the taser would give the suspect relief and therefore lead them to now stop fighting with the police.