The Three-Fold Law is the belief and principle on which Magick is used. This law relates to the use of power and energy, for when used, power is returned to the sender, three times the level it was sent out.
Used in relation with the ethos, "Do what thy wilt, though it harm none" (as stated in the Wiccan Rede), witches take great care when preparing and casting spells that no harm should come to others because of it.
The nature of this three-fold return can perhaps be better understood when considering the cause and effect principle of a spell. Let's take an example: Someone comes to you in distress with a problem. You help by sending out positive energy and the problem is successfully resolved. This automatically makes you feel good about yourself and influences your thoughts; thus your state of consciousness is altered.
In turn your thoughts stir the emotions; which has a physical effect in the body by causing glandular secretions to enter into the blood stream. The resulting state of mind and body lifts your spirit, which is what makes you feel good. Thus, with the act of a single good deed you have been effected three times in mind, body, and spirit. This then is your three-fold response to the return of positive energy. Now consider your response had you sent out negative energy or something went drastically wrong.