Reinforcement is how the brain remembers important learning experiences and associates stimuli with appropriate decisions. Positive reinforcement raises the probability of recent decisions given recent stimuli, while negative reinforcement decreases it.
Reinforcement is mediated by a chemical that is produced shortly after any rewarding or punishing experience. In Creatures 1 and 2, it strengthens recently-active connections in the decision pathway, which makes them less likely to migrate. As a result, the concepts that were used to trigger the rewarded or punished decision become less likely to be forgotten, as is the relevance of that concept to the decision that was made. It has a very short half-life so that unrelated responses are not accidentally reinforced.