Iโm an expert on Nietzsche (Iโve read some of his books), but not a world-leading expert (I didnโt understand them). So take all of this as a riff on the concept, rather than a guide to Nietzscheโs original intent.
In the beginning (says Nietzsche), the word โgoodโ was synonymous with โnobleโ - ie the virtues that made the nobility better than the serfs they ruled. This was way back in the Bronze Age, so your model for a noble should be Achilles, Agamemnon, etc.
The excellent noble delights in being strong, healthy, and virile. He lives in a beautiful palace and wears shining golden armor. He may be cultured, sophisticated, or even brilliant. Heโs great at everything he does, and harbors ambitions to become even greater, maybe conquer a kingdom or two. Heโs powerful, skillful, and awe-inspiring. Life is good!
Value systems naturally flow from elite to commoners. But a commoner canโt do much with this kind of master morality besides conclude โyeah, I suckโ. Commoners are poor, sickly, and live in mud huts. Theyโre unlikely to achieve many goals beyond โnot dieโ, and theyโve probably had their spirits crushed. But โI suckโ isnโt a psychologically stable proposition. So sometime around the Iron Age, the slaves started working on a morality of their own, one where theyโre the good guys and the masters are the losers.
Slave morality says that the strong are tyrants, the rich are greedy, and the ambitious are puffed-up braggarts. The wisest man is he who admits he knows nothing; the strongest man is he who conquers his own desires; it is easier for a camel to pass through a needle and so on. God loves the humble, the salt of the earth. The worst thing you can do is try to pridefully rise above your fellows (cf. Tall Poppy Syndrome); the best thing you can do is to lessen yourself, through methods sacred (fasting, celibacy, self-flagellation) or mundane (giving to charity, serving your fellow man).
Nietzsche speculates that slave morality originated with the Jews (an especially downtrodden and persecuted race) but caught on after the rise of Christianity. Sometime around the fall of Rome it took the lead over master morality, and itโs been gaining ever since. As time goes on, slave morality will become more and more dominant, master morality will fade into a dimmer and dimmer memory, and at some point weโll come to what he calls the Last Manโsomeone so completely poisoned by slave morality that he worships mediocrity, feels no emotion but envy, and refuses to ever do anything because doing things seems insufficiently humble.