It’s a necessary, but not a sufficient, moral law.
It is the negative form of the Golden Rule, don’t do to others what you wouldn’t have them do to you (i.e. harm you).
It’s necessary, i.e. you can’t be moral without following it. But it’s not sufficient, i.e. following this rule ALONE is not enough to make you a moral person, from a Christian standpoint.
The other question which the statement begs is the definition of ‘do what you will’, is the will something more than the result of one’s passions, or does ‘do what you will’ mean the same as ‘do what you want’?
(Short note on its’ provenance - though often associated with Wicca, this rule basically derives from the 19th century liberalism of J S Mill, and was taken up and turned into a commandment by the 1930s English middle-class Wiccan founder Gerald Gardner.)