Reinforcement is an Operant Conditioning term that refers to a process by which the likelihood of a behavior occurring is increased either by giving a pleasant stimulus (positive reinforcement) or removing an unpleasant stimulus (negative reinforcement).
Imagine I want to teach my dog to sit on command. When I tell him to sit and he does, I give him a treat. Giving the treat increases the probability that he will sit the next time I tell him to sit.
Check out the definitions for Positive Reinforcement and Negative Reinforcement, as well as our Learning and Conditioning topics page for more information.