system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What in the hell is "Postmodernism?" 

And for bonus points: "Dadaism?"
gpt
Postmodernism is a cultural artistic era, it's kind of a label to put on works like the Renaissance or the Medieval period, but much much more recent. 

I think it's best explained through literature: Previously, you may have a story that tied up nicely, and the ending made sense. But after World War II and into the Cold War, it didn't seem like things were making sense anymore, and a lot of our history seemed a bit absurd.  So, "postmodern" literature began rejecting the idea that things could be tied up so neatly. The "postmodern" era of literature is filled with unreliable narrators, fragmented stories, and a lot of irony. This is reflective of the larger cultural feelings of relativism (i.e., there's no one Truth with a capital T, it's just a matter of where you are in the world) and a recognition of the absurdity that was going on. 

Some now say we are in the "post-post-modern" period, but it's not like all of the artists/cultural contributors get into a room and decide what era the calendar page is on. Usually people are just influenced by their generation, the history they are writing in, and the culture around them.