“I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do.”
— Robert A. Heinlein
This, in essence, is just a restatement of “do what thou wilt shall be the whole of the law.” As noted by @SmashTheState, it is an assertion that externally delivered rules cannot ever have the authority that rules we take on for ourselves can have. Such an idea unites as diverse a group of people as Kant, Crowley, Nietzsche, and Heinlein. Whatever one thinks of it in the final analysis, then, it is an idea worth taking seriously.
And since it is my professional duty, a few notes on Nietzsche:
(1) Nietzsche despised both anti-Semitism and nationalism. Both views come in for a great deal of criticism in his major works, and Nietzsche spent a good amount of effort trying to get his sister to renounce her nationalistic and anti-Semitic tendencies.
(2) The Nazis didn’t actually read Nietzsche. Hitler’s favorite philosopher was Schopenhauer, whose work he kept with him at all times while serving in World War I. The propagandists assigned to redact Nietzsche’s work to make it look like he supported the Nazi cause considered their job a hopeless task and wound up having to falsify entire passages to make the claim.
(3) The Nazis tried to appropriate every famous German to their cause, no matter how far in the past and how opposed to the tenets of Nazism that thinker might have been. That they tried to appropriate Nietzsche is no more indicative of what he was about than the fact that they tried to appropriate Goethe (and Jesus, by the way).
Indeed, the Nietzsche-Nazism myth has been debunked so many times, I can’t figure out why anyone still believes it at all.
As for his major ideas:
(A) The death of God is the idea that Europeans were facing a crisis due to the rise of science making it hard to take God seriously as an explanation for anything in the world, thus undermining the ordering principle of their culture. Specifically, his point was that ethical systems would have to be completely revised if belief in God is rejected.
(B) Perspectivism is the view that no human being has the complete truth, and that we can only approach what is actually the case by taking many different viewpoints into consideration. It does not say that there are no objective truths, it does not say that no viewpoints are better than others, and it does not say that truth is relative.
(C) The Übermensch is not actually a central idea to Nietzsche’s philosophy and is discussed very little in Nietzche’s completed works. The main characteristic of an Übermensch is that he has overcome nihilism: he lives a life that he finds meaningful rather than lamenting that the world does not impose meaning upon him.
(D) Amor fati, or the love of one’s fate, is about living a life without regrets. It is connected to the idea of the eternal recurrence, which is a thought experiment in which we are asked to imagine a world in which we live the same life over and over again. Nietzsche does not say that this is how the world actually is, but says only that a good life can be recognized by whether or not we would be able to affirm living it on endless repeat in this way.
(E) Finally, the will to power is most fundamentally about self-control as a virtue. We have competing drives, according to Nietzsche, each of which is competing to be the guiding force of our lives. To live well, he thinks, we must overcome our less worthy drives and place in the position of power that which we have decided is the one most authentic to who we want to be. This requires immense self-control, as our other drives are quite strong and insistent on getting their way.
These are all simplifications, of course, as each idea could be the subject of a book-length dissertation. Still, it is my frequent observation that the average discussant of Nietzsche has no idea what he actually said about anything.