A LOT of people are excited about this, so I'll check it out.
Gawain is attending King Arthur's court when the Green Knight comes in and says "hit me and I will hit you in return, and everything will be fine" and Gawain hasn't seen the script so kills the Knight. Cut to, his journey later to go to the knight and get the returned blow.
This is based on classical mythology and... I don't get it. I wonder if I knew the story more I would? There are likely lots of references and such that makes people go "hey, that's that thing!" but none of that meant anything to me.
This movie is definitely artistic, with lots of stylish flourishes. But again, while I could appreciate it as a technical production, again the story didn't really resonate with me.
What this comes down to is: it's a nice movie, I guess, but not really meaning much to me.