Reinforcement is when a behavior is increased or maintained by its consequences. Reinforcement may be either positive or negative. The term comes from behavioral psychologists -- those who focus on the factors that cause and maintain behaviors (or actions).
Positive reinforcement occurs when a behavior is increased because it is followed by the presentation of something desirable. For example, a child who is praised each time he puts his clothes in the laundry will be more likely to clean up his clothes again in the future.
Negative reinforcement occurs when a behavior is increased because it is followed by the removal of something undesirable. Some experts believe that this is why self-harm and other impulsive behaviors are often maintained in people with borderline personality disorder (BPD).
For example, imagine that a triggering event such as an interpersonal rejection causes you to have a strong emotional reaction (e.g., fear and anger). If you engage in an impulsive behavior (e.g., binge drinking alcohol), the emotions are reduced and you feel better in the short term.
In other words, the impulsive behavior resulted in the removal of the undesireable event (the emotions). As a result, the impulsive behavior is more likely to occur the next time you encounter emotions (even if the long term result of the impulsive behavior is not healthy).