The important concept of ressentiment appears frequently in Nietzsche's writings. This French word is pretty much equivalent to the English word "resentment," and Nietzsche uses it largely because there is no German word for "resentment." It is the central creative force behind Nietzsche's conception of slave morality.
We might better understand the slave's ressentiment by contrasting it with the contempt felt by the master toward the slave. In Nietzsche's view, the "bad" of master morality is an afterthought for the masters' that does not much concern them. They look down on slaves with a shrug of contempt: the thought process rarely goes beyond "sucks to be you." By contrast, the slaves' ressentiment for their masters is a consuming passion, one that poisons them and makes them bitter. Quite contrary to the passing contempt of the masters, this ressentiment is a primary focus of the slaves' energy and attention.
In a sense, the life of the noble man is much simpler: nothing ever stays with him for too long. If he's upset, he lets it slide, and if he's happy, that happiness is a present happiness: the noble man lives in the present.
By contrast, the man of ressentiment allows things to build within him. Injuries against him slowly build as resentful hatred, and he constructs happiness through long thought processes. Because his focus is never on the present, the man of ressentiment also builds hope and cleverness in a way the noble man does not. According to Nietzsche, all this thought and hatred culminate in the invention of the concept of evil and the denotation of the noble man as "evil."
Nietzsche's criticism of slave morality is largely based on the fact that it develops out of hatred, denial, and an evasion of present realities. The hopes of the slave are on a promised afterlife, the focus of the slave is on people who hardly even think of the slave: there is no emphasis on the self or on the present. As a result, a contemporary Europe that has been infected with slave morality has become insipid and dull, having given up all sense of ambition for itself and for the present.
The commentary on the previous section suggested that Nietzsche did not make moral valuations himself, and yet here he seems to be coming down very harshly against slave morality. Perhaps we should refine that remark by suggesting that while Nietzsche does judge systems of morality, he does not judge them from the vantage point of a particular preferred system of morality. His judgments are not based on particular values that he admires or detests so much as they are based on a vision of the kind of person created by the different systems of morality. Nietzsche despises slave morality for the manner in which it devalues life. Because ressentiment brings our attention away from ourselves and away from the present, we become less creative, less assertive, and less motivated. It creates people that are no longer driven to improve themselves.