Positive reinforcement is the concept of adding something to a person's environment. It is not always something you would consider to be positive in nature. For example, if someone is screaming their head off in order to get attention and you yell at them to stop, you are positively reinforcing the behavior by giving them that attention. Now they have learned that if they want attention, screaming their head off will get them attention and they are more likely to use these means again the next time they want attention.
The concepts of positive and negative reinforcement are often misunderstood. One is naturally prone to think that getting what we would consider bad results, such as yelling at the person, as negative reinforcement. An example of actual negative reinforcement would be a case where you are listening to music that someone else in the room doesn't care for. They scream their head off and you turn the music off. You removed the stimulus of the annoying music, negatively reinforcing the screaming the head off behavior. If they ask you politely to turn the music off and you turn it off, you are now negatively reinforcing the polite request.
"Screaming my head off gets the music turned off," is negative reinforcement of screaming one's head off.
"Screaming my head off gets the music turned on," is positive reinforcement of screaming one's head off.
Positive reinforcement is often misunderstood both as a behavior management technique and in day to day life. We don't think we are positively reinforcing maladaptive, or bad, behavior with the things that we do. We tend to think that doing something that makes someone stop behaving in an undesired way helps them to stop behaving in that fashion. Something as simple as giving someone a hug because you know that will stop them from being sad can actually reinforce their sadness if the person has no other means by which they can get hugged.
The trick in behavior management is to reward positive behaviors with the desired reinforcement for maladapative behaviors. A person wanting attention who is used to getting it by having tantrums or wetting his pants can learn there are other ways to get attention, such as stating they are upset and want help dealing with those feelings or by asking to go to the bathroom. Behaviors exist for reasons, often as a ways to receive stimulus, whether it is attention, rewards (what constitutes "rewards" are in the eye of the beholder), or if the behavior is self-stimulating (it can be a lot of fun to sit and scream your head off).
So, the next time you react to someone's behavior, especially if it annoys the living crap out of you, consider how you are reacting and what that reaction provides the other person. You could be positively reinforcing that annoying behavior without even knowing it.