Taken from rationalwiki.
"Postmodernism is a cluster of philosophical, literary, cultural and art movements which developed in the mid-20th century. It developed out of philosophical criticisms of modernism and disillusionment which resulted among European philosophers on the political left following World War II, when the communist governments of Eastern Europe became increasingly unable to conceal their misbehavior, thus suggesting to the intelligentsia that Marxism was not the greatest thing since sliced bread.
Postmodernism is, per its name, a reaction to modernism, wherever a postmodernist feels something that could reasonably be tagged that needed reacting against; this means that it's not one coherent thing. It also means a lot of it is difficult to understand unless you understand what it's a reaction to.
Contrary to various rationalists' views, postmodernism is not composed entirely of bullshit -- it is a useful critical method to keep on hand when talking about social and artistic things, like most of what humans do that might be called "culture." Humans are ridiculously full of shit, and postmodernism can be useful in pointing out its effects. However, as the product of humans, it is itself susceptible to bullshit in turn, particularly when applied to actual reality rather than human conceptions of reality."