If you're a buff of the Arthurian legends -- what, you mean every 9-year-old wasn't checking out "Le Morte d'Arthur" from the public library? -- the tale of Sir Gawain and the Green Knight is well known to you. It's a parable about bravery and honesty, as one of the most steadfast knights is faced with a strange challenge and finds himself lacking.
"The Green Knight" is far from what most people expect of a King Arthur movie. One of my all-time favorite films is "Excalibur," one of the first R-rated movies I ever got to see, and this movie from writer/director David Lowery is decidedly not that.
It's a moody, existential exercise that has very little action or traditional narrative. It's written and directed by David Lowery, who has made mainstream fare like "Pete's Dragon" but also "A Ghost Story" and "Ain't Them Bodies Saints," dreamy films that are almost experimental in their eschewing of usual forms of storytelling.
It's visually resplendent, dark, deliberately hard to decipher. You will occasionally have trouble understanding what people are saying or what is going on. It's not so much the kind of movie that you watch and enjoy as an experience that slowly absorbs into your skin.
Imagine if Terrence Malick made a Knights of the Round Table movie -- that's the best description I can give.