The Law of Return is one of the last couplets of the Wiccan Rede:
"Mind the Threefold Law ye should,
Three times bad and Three Times good."
This means that all you do will come back to you. So if you do something good in your life it will come
back to you threefold, but if you do something bad in your life, then that to will also come back to you threefold.
This rule / law is basically telling you to harm no one, which sums up the last line of the Rede:
"And ye harm none, Do what ye will."
Meaning, in Wicca, as long as you hurt no one, you can do you want. This is the most important law /
rule in Wicca.