Nietzsche argued that a distinction existed between the morality of masters or the nobles of a society and those of their slaves, or those who are poor and powerless compared to the masters. Master morality is defined as "good" by the masters, because they see themselves as "good" because of the power that they have. The masters look at the slaves and see that the slaves are characterised by the opposite of what characterises them as masters, and thus think that the slaves are "bad." Master morality is thus defined by the contrast between what is seen as "good" and "bad" as defined by the identity of the masters.
Slave morality, by contrast, is something that is developed by the "slaves," or those who are poor, powerless and weak compared to the masters or nobles. They resent and hate the status and power of the nobles and masters, and think of them as being "evil" and see themselves as "good" because of the imbalance of power between them. Slave morality is thus defined by what is seen as "good" and "bad" as defined by the weak and powerless of a society.
Nietzsche argued that slave morality would always triumph over master morality because slave morality, based on the resentment that drives it, caused the "slaves" to become more intelligent than the masters and spend far more time nursing their grudges and resentment. Note the following quote from Nietzsche that explores the difference between slave morality and master morality:
To be incapable of taking one's enemies, one's accidents, even one's misdeeds seriously for very long—that is the sign of strong, full natures in whom there is an excess of the power to form, to mold, to recuperate and to forget... Such a man shakes off with a single shrug many vermin that eat deep into others; here alone genuine 'love of one's enemies' is possible—supposing it to be possible at all on earth. How much reverence has a noble man for his enemies!—and such reverence is a bridge to love.—For he desires his enemy for himself, as his mark of distinction; he can endure no other enemy than one in whom there is nothing to despise and very much to honor! In contrast to this, picture 'the enemy' as the man of ressentiment conceives him—and here precisely is his deed, his creation: he has conceived 'the evil enemy,' 'the Evil One,' and this in fact is his basic concept, from which he then evolves, as an afterthought and pendant, a 'good one'—himself!
Having a "strong, full nature" as a master is actually a negative, Nietzsche argued, as it meant that you could forget the injustice and evil of others. By contrast, slave morality by its very definition causes the "slaves" to linger on their resentment and to let it fester into acts that will eventually overcome master morality.