postmodernism isnt always the same from person to person, but there are trends that follow from writers in the immediate post-war era
basically a "rejection of universalist notions of objective reality, morality, truth, human nature, reason, language, and social progress" if we go by wikipedia
there is no grand universal standard that applies to the whole world from these, everyone perceives things differently
its about questioning the hierarchy, getting the audience to be more aware, active instead of passive
the simplest possible example of this attitude is subversion, because it is supposed to get you to engage in a discussion about what was subverted and why
instead of a knight rescuing a princess from a dragon, its the princess realizing the knight sent to save her will be killed by the dragon and has to save him
its asking you to question the status quo of storytelling instead of just going "here is ye olden story, dont ask how a tiny knight can kill a big fire breathing reptile with only a sword"