"Positive reinforcement" is a term from psychology, from classical conditioning as practiced by Pavlov and Skinner. You change a subject's behavior by applying reward (positive reinforcement) or punishment (negative reinforcement). In your excerpt, a person is trying to change his own behavior by eating better, and the writer suggests that the good feeling the person will get from suddenly not eating any bad stuff will act as a reward that will work to change his behavior.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.