This middle English poem is said by some to be the greatest poem of Middle English literature, however it does have to compete with The Canterbury Tales for that title, something that I am not going to go into since I have not have the chance to read Chaucer's work at this stage. However while the earliest copy of this poem exists on a manuscript dating back to 1400 AD, it was probably circulating much earlier than than. We don't actually know who the author of this poem is (and my suspicion is that it is like The Odyssey - it was an oral poem what was written down at a later date, and copied, and the version that we have is the earliest version of this copied text). The manuscript also contains two other poems, the Pearl and Sir Orfeo (both of which Tolkien translates as well).
While the poem begins in King Arthur's court, during a feast, the opening stanza goes back to the founding of England - Troy. It appears (and I noted that Holinshed, the English Chronicler from whom Shakespeare used as a source for some of his plays) also puts the origin of the British people to Troy. The story is that after Troy fell, Aeneas fled to Italy where he founded a colony and from that colony Romulus and Remus arose and went on to found the city of Rome (or at least Romulus, since since he killed Remus). However, one of Aeneas' general's, Brutus, was not happy with the location, so he left with some followers, sailed to Britain, defeated the Giants, and founded a colony that eventually, after millennia, went on to rule the world. This of course is all legend and there is no historical or archeological evidence to support this ever actually happening (though it does make a ripping yarn).
Anyway, that is beside the point. The poem itself was quite popular and tells the story of Sir Gawain who, at the feast, decides to take the Green Knight's challenge, which is that if somebody where to strike him then they must meet him at the Green Chapel in a year and a day and also be struck. Sir Gawain decides that sounds like a bit of fun and proceeds to lop off his head. However the Green Knight simply picks it up (his head that is) and walks out, telling him that he will see him in a year and a day. So Sir Gawain travels the land and arrives at the castle of Sir Bertilak. Bertilak then heads out out on a hunting trip but before he goes he tells Sir Gawain that if he gives Gawain the proceeds of the hunt, Sir Gawain must give him whatever he got that day. So, while Bertilak is out his wife attempts to seduce Gawain, who resists the temptations, and the first two times he is given a kiss, and the last time he is given a girdle which will protect him from harm. Gawain honours his agreement to Bertilak (with the exception of the Girdle) and then goes out to meet the Green Knight. After the battle, it turns out that the Green Knight is Bertilak.
This poem carries a lot of symbolism which will simply take way too long to explore the subtleties (and for those who are interested, I'll simply refer you to Wikipedia). However, one of the major themes in this poem is chivalry, which is a medieval code of honour for knights. One of the major aspects is honesty and keeping one's words. Gawain does demonstrate his honour by keeping his word to Bertilak (and the Green Knight) however he does fail with handing over the girdle. This is interesting because he keeps the girdle for protection when he meets the Green Knight without actually knowing that he has already met the Green Knight. We also notice that he honours the marriage vows by resisting Bertilak's wifes advances, even though each advance becomes progressively stronger. Some have suggested that the advances of Bertilak's wife reflects Bertilak's hunting trips, as the animals Bertilak hunts become progressively more aggressive.
Another aspect is nature verses civilisation with the Green Knight (and in turn Bertilak) representing nature and Camelot representing civilisation. The Green Knight's entrance to the feast is a reflection of the chaos of nature bursting into the order of civilisation, and Sir Gawain takes the challenge in an attempt to tame nature. However, considering the time this poem was written (the Dark Ages) it is also looking back to a more civilised time (despite doubts as to the actual existence of King Arthur's court). The time in which this poem was composesd was a time of lack of law and there is, in a way the hearers were no doubt longing for the better times.
In this particular work, there are two other poems which I will briefly mention. The first is The Pearl, which is an allegorical dialogue between a knight and a woman about the kingdom of heaven. This poem has a lot of biblical images (which includes the Pearl, a symbol that Christ uses in one of his parables to describe the beauty and value of the kingdom of heaven). The Knight had the pearl, but lost it, and is worried that in losing the pearl he has also lost access to the kingdom of heaven. Some have suggested that the Pearl is probably representing a loved one, such as a child, but I think the allegory in this poem is deeper than that.
The final poem, Sir Orfeo, is a retelling of the Greek story of Orpheus in the underworld (and this is very clear in that Sir Orfeo is a form of Orpheus). The poem is set in Winchester, however in the poem it is also called Thrace (the location in Greece of the original Orpheus myth). In this poem, though, instead of travelling to the underworld to rescue is beloved, he travels to the Faerie world where his beloved has disappeared. However, in all forms, this poem is the same as the Greek legend.