A law that is not consistently enforced does not serve its purpose. Law without enforcement is not law; it is merely statute -- a promise of law. To institute real law is not merely to declare that such and such behavior is forbidden; it is also to punish those who violate that edict. Furthermore, those who enforce law must punish without favor for their friends or malice for their enemies. To punish only those one dislikes while forgiving others is not to enforce law but to engage in the arbitrary and unjust exercise of power.
The main point of the passage is that instituting real law consists in clearly defining unacceptable behavior.