Postmodernism is a loose coalition of diverse thinkers from several different academic disciplines, so it is difficult to characterize postmodernism in a way that would be fair to this diversity. Still, it is possible to provide a fairly accurate characterization of postmodernism in general, since it friends and foes understand it well enough to debate its strengths and weaknesses.
As a philosophical standpoint, postmodernism is primarily a reinterpretation of what knowledge is and what counts as knowledge. More broadly, it represents a form of cultural relativism about such things as reality, truth, reason, value, linguistic meaning, the self and other notions. On a postmodernist view, there is no such thing as objective reality, truth, value, reason and so forth. All these are social constructions, creations of linguistic practices and, as such, are relative not to individuals, but to social groups that share a narrative.
Postmodernism denies the correspondence theory, claiming that truth is simply a contingent creation of language which expresses customs, emotions, and values embedded in a community’s linguistic practices. For the postmodernist, if one claims to have the truth in the correspondence sense, this assertion is a power move that victimizes those judged not to have the truth.
He is not a fan. Not at all.