Postmodernism (sometimes spelled "post-modernism") was a late-twentieth century cultural movement that pushed back against some of the assumptions made by Modernism, which came just before it.
In particular, postmodernists were unimpressed by modernism's claims that a universal truth—or a universally-understood way to express that truth via art or philosophy—even existed.
Postmodernists instead began to explore the ways in which knowledge is culturally based and often fragmented or biased. Some postmodern thinkers went so far as to argue that we can't "know" anything, because the tools we use to "know" things are themselves things we invented from our own fragmented, biased worldview.
Embracing the idea that no viewpoint can ever be universal gave rise to fields like feminist theory, critical disability theory, race studies, and deconstruction.
It also changed artists' and writers' approach to their work. Rather than trying to express some kind of universal truth through art or literature, artists and writers accepted that every point of view was incomplete. They started playing with art and narratives that depended on point of view, like art pieces that change depending on the angle you view them from, or works of literature where the story is told through differing characters' perspectives.
Some artists went so far as to blur the line between art and parodies of art, or between what we think of as "art" and what we think of as "not art." Perhaps the best example is John Cage's 4'33". It's a musical work in which the musicians play no instruments; the "music" is whatever ambient noise happens during the four-minute, thirty-three-second interval the piece is performed.
Modernists would have said 4'33" was definitely not music. Postmodernists respond by challenging the very definition of "music."