From Creatures Wikia
Reinforcement is the mechanism of how the brain comes to be conditioned to associate stimuli with a response. Positive reinforcement (like rewarding) is used to condition a positive association while negative reinforcement (punishing) is used to stop or discourage an association from forming.
Reinforcement is modelled in the Creatures series by a chemical which is produced as a result of decay of the reward or punishment chemicals. It allows, for example concept space dendrites to become stronger. It has a very short half-life so that unrelated responses are not accidentally reinforced.