Actually the Wiccans adopted and incorporated this saying into their system, the phrase was from Aleister Crowley who used it in the Thelemic system. Liber Al Vel Legis is the Book
This Book lays down a simple Code of Conduct. "Do what thou wily shall be the whole of the Law."
"Love is the Law, love under will."
"There is no law beyond Do what thou wilt."
This means that each of us stars is to move on our true orbit, as markets out by the nature of our position, the Law of our growth, the impulse of our past experiences.All events are equally lawful-and every one necessary, in the long run-for all of us, in theory ; but in practice, only one act is lawful for each one of us at any given moment. Therefore Duty consists in determining to experience the right event from one moment of conciousness to another.
Each action or motion is an act of love, the uniting with one or another part of "Nuit"; each such act must be 'under will,' chosen so as to fulfil and not thwart the true nature of the being concerned.
The technical methods of achieving this are to be studied in 'Magical,' or acquired by personal instruction from the Master Tgerion and his appointed assistants."
Well, the "Thou" there represents the higher will, which is developed by bringing asll of the parts of the lower will under its authority through the alchemical processes of separation, purification and cohabatation. When you function under this Will, you follow the Law.
Love is the Law, Love under Will.
This speaks to the nature of the Law, and of the Will. The Higher Will is guided by a unprejudicial love for all of creation because it recognized the oneness of all of creation, manifest and unmanifest.
I would just like to add the love is union, that is the marriage of two seemingly separate things to create one whole unit. That is, to achieve a state of unity or oneness, to get away from separation and to realise that we are all fragments of the one universe surfing on the continuity of existence.
So "love is the law, love under will" signifies that the law is to acheive union with all of creation under the higher Will of the genius/holy guardian angel (call it what you Will).