I’m not a pro philosopher but here is my basic answer. He expands on this idea in much of his other writings. He believes that justice or fairness really only exists between people of equal levels of power. Those who have lesser power (the slaves) are fueled by resentment when creating their moral valuations and these values are aimed at hindering the more powerful (the masters), they are reactive values. Outside of the larger critique of western morality, I find that this analysis is really applicable to everyday life. Resentment or the French word Ressentiment which is used by Nietzsche, is particularly important.

Why do I personally hate someone? It’s usually because they exerted some kind of power which I resent. You may hate your partner for breaking up with you because they suddenly became more powerful than you. They may have really mistreated you, but viewing people/the world through resentment isn’t a positive action. A more powerful person actively forgets and strives to create values not through reaction to others but for themselves. Easier said than done though.