A significant and fundamental Nietzschian concept. It can be found discussed extensively in his famous work On the Genealogy of Morals, as well as in Twilight of the Idols. The basic idea, from its beginnings, is given here. In order to get the full extent of Nietzsche's arguments, though, one must read his words on the subject...

Originally, according to Nietzsche, man's system of morality was based on the concept of weak vs strong: the weak were seen as evil, and the strong were seen as good. This was especially true in Hindu culture, where the powerful caste of the Brahmins were seen as good, and the lowest, weakest caste, the chandalas, or untouchables, were seen as evil (and this justified their being at the bottom of the social order). This could also be seen in the ancient Greek and Roman cultures, which were essentially based on a morality of strength = goodness. This morality Nietzsche calls the noble morality, or the morality of action.

Eventually, however, this "noble" morality was replaced by Judeo-Christian morality, which, according to Nietzsche, is a morality of weakness, or "slave morality," so called because it sees strength as being evil, and the oppressed, suffering people as being good. For support is the famous saying "The Meek shall Inherit the Earth" (from the sermon on the mount).

Nietzsche is obviously opposed to the "slave" morality. Why? Firstly, it is, rather than an affirmation of life, a denial of it. This system of morals teaches us to deny our instincts which to Nietzsche is dehumanizing. It teaches us that our instincts are evil; Nietzsche argues that it sets up an ideal (embodied in Christ) that no human being can achieve. Indeed, nobody should want to achieve it, since it is a denial of what makes us human, what truly defines ourselves. Secondly, it is a morality based entirely upon guilt and fear. We are taught to follow God's commandments because if we do, He will reward us greatly in the afterlife, and if we do not, we are punished in hell for all eternity. This teaching makes us feel incredibly guilty all of the time, for our actions which may violate some law of God's. Indeed, "all religions are at the deepest level systems of cruelties."*

With this bleak outlook on the Christian morality, Nietzsche asks the question: what truly is the point of all of these values? Do we really need them at all? Or are they simply damaging to the human spirit?

Nietzsche argues that we do not. It is this enforced morality itself that has caused man to be "evil," in this sense, i.e., to need to be controlled by these morals. "It is society, our tame, mediocre, castrated society, in which a natural human being, who comes from the mountains or from seafaring adventures, necessarily degenerates into a criminal."+ Without this system of morals, man, who is naturally good when he is free, will be happier, and stronger. On whether or not it is possible for society to cast off these morals, Nietzsche says:

Is this even possible today? But some day, in a stronger age than this decaying, self-doubting present, he must yet come to us, the redeeming man of great love and contempt, the creative spirit whose compelling strength will not let him rest in any aloofness or any beyond, whose isolation is misunderstood by the people as if it were a flight from reality, so that, when one day he emerges again into the light, he may bring home the redemption of this reality: its redemption from the curse that the hitherto reigning ideal has laid upon it. This man of the future, who will redeem us not only from the hitherto reigning ideal but also from that which was bound in to grow out of it, the great nausea, the will to nothingness, nihilism; this bell-stroke of noon and of the great decision that liberates the will again and restores its goal to the earth and his hope to man; this Antichrist and antinihilist; this victor over God and nothingness - he must come one day. ||

* Nietzsche, On the Genealogy of Morals, translated by Walter Kaufmann, pg.61
+ Nietzsche, Twilight of the Idols, translated by Richard Polt, pg.80
|| On the Genealogy of Morals, pg. 96

On the Genealogy of Morals and Ecce Homo, by Friedrich Nietzsche, trans. Kaufmann, Hollingdale. Vintage Books, 1989.
Twilight of the Idols, by Friedrich Nietzsche, trans. Polt. Hackett Publishing, 1997.