A concept which Nietzsche talks about extensively, slave morality is a values system that opposes master morality, it is a view and act of revenge against the strong. Slave morality praises weakness and condemns strength, devaluing what the master values and what the slave doesn’t have.
The meaning of “good” is the antithesis of the original aristocratic “good”. It’s an inversion of the values and develops out of the resentment that the weak feel toward the powerful.