"Do not plant in your field what you do not wish to collect."
That saying in itself basically sums up the Three Fold Law.
If you do something good, it will come back upon you thrice as great.
The same goes for if you do something bad: thrice back upon you.
It's a much stronger version of Karma, hence stopping (or it is supposed to stop) witches that are plagued by grief, anger, or bad feelings from casting spells or curses that will harm another being for fear of the pain returning onto them.
The only reason one should ever cast a spell to directly harm someone or something is if they or someone close to them is in life threatening danger.
And even then you can indirectly harm, block, ward, or bind someone from harming or doing something bad, by, for example, wishing that no good luck comes their way, instead of casting a bad luck spell.
This in term brings us back to the Wiccan Rede: "An ye harm none, do as thou wilt."
The Wiccan Rede is the law, in a sense, and the Three Fold Law is, ultimately, the punishment for breaking this law.
