Nominalism is a movement within the late Medieval (Western/Latin/Roman) Church that stressed the priority of will over intellect (or mind). It is usually associated with the names of William of Occam (famous for Occam's Razor: "entities should not be multiplied beyond necessity") and Duns Scotus. It is called Nominalism because the names used to signify thins and qualities are just that -- names. These common names for qualities and things do not refer to universals (i.e., there is no transcendent idea of blue unifying the multitude of differently blue things, there is no transcendent idea of color to unify the multitude of different colors, etc.). It was supposed by earlier Medievals (following the pagan Platonic tradition) that there must be a transcendent idea of color, otherwise the unity of the many intelligible colors would fall apart: what is the ground of calling all these colors colors if they are not unified in a timeless intelligible idea of color-as-such? For Nominalists, "color" or any other so-called "universal" is simply a name, something we do with our tongue and vocal cords and air: the names are convenient shorthands for organizing our experience into unities that only exist in our minds and in our speech -- these unities do not exist out there.
This also applies to the concept of the Good.
In terms of what that means for what things are (ontology), this means that there are no common natures -- you and I do not share a common human nature, but are rather radically individual bodies; politically speaking, this means that there is no common good: the vocalization "good" is simply the name we give to certain kinds of external conformities of will: if the will of the king is that you rise at five AM to offer incense to an image of him, then doing that is good, and not doing that is bad (unless there is a higher, divine will that trumps the king's).
Now, this means that the mind cannot discern what is good transcendent of positive wills involved. There is no transcendent Good independent of the wills of men and the Will of God. God, in this model, can even will to hate Himself, and will that His creatures do the same, and then change it -- because there is nothing above God, in this model, and there is no idea of the Good internal to His Nature.
One can see how this leads to a horrible kind of Nihilsm. It also, I think, means that there cannot be philosophy in any classical sense of the term, but instead there can be only ideology.
Under such a conception of life and human wills, the only option, for those whose wills are set to peace, for those whose appetite is for traquility rather than conflict, is to admit that there are multiple ends that variously satisfy and fulfil many different people, and that there is no singular end of human life that is natively the good of human life as such that can be thus commended to ("foisted on") others, and to recommend a kind of pluralism.
Nominalism + Appetite for Peace = Liberalism.