system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Postmodernism

EDIT: What's with the mass downvoting going on in this thread? :/
gpt
There were a group of artists and writers that grew up during the first large scale wordwide modern war (WWI) and also lived through the second World War.  This generation of creative people witnessed so many horrible things that the art that they made was really trying to make sense of all those awful, evil parts of life.

These writers and artists are called "modernists" and they really invented a lot of the ways that we tell stories today and how we decide whether something is a good or bad story.  This is because the "modernists" really wanted to unite all of the evil they had experienced, with all of the good they knew that people could do.  After all, these people saw so much killing and war, but they also saw people inventing so many wonderful new things.  Think of if people were all riding horses now, but by the time you're finished with High School most of us have cars!

It was too bad though that none of the "modernists" ever really felt like they *were able* to tell one story that explained all of the bad things and all of the good things together.  They never figured it out, and thought that they failed.

The artists that came next are called "post-modernists" only because they came *after* the "modernists."  They saw that the "modernists" weren't able to come up with a story that explained all the good and bad stuff humans could do, so they decided that there was *actually no way to explain it*.  

That's why a lot of "post-modern" artists don't really try to create anything that is totally new and original.  They figured that if the "modernists" tried so hard to tell this story they've pretty much tried everything that can be tried.  The "modernists" wrote every book, painted every picture, and sang every song trying to "figure out" what life was all about.

tl;dr (and you're over 5): Modernists tried to unite humanity with one grand narrative, but couldn't.  Post-modernists decided the reason that they couldn't was because there is *no* way to unite the entire story of humanity in a grand narrative.  Since there is no way forward we're stuck just regurgitating the art of the past, forever.

"Post-modernism" is based on "quoting" and using other works of art as a way of coming to terms with the fact that there probably *is* no way that we can explain why people can do so many great things, but can also do so many really bad things at the same time.