The Fisher King legend appears in TWOT in several guises, the first and most obvious being the unhealable wound in Rand's side which he received from Ishamael's staff in [TGH: 47, The Grave Is No Bar to My Call, 564]. Then, we have the header prophecy from ACOS: "There can be no health in us, nor any good thing grow, for the land is one with the Dragon Reborn, and he one with the land." Finally, we have the "Fisher" piece in Moridin's sha'rah game [TPOD: Prologue, Deceptive Appearances, 44]: "The Fisher was always worked as a man, a bandage blinding his eyes and one hand pressed to his side, a few drops of blood dripping through his fingers. The reasons, like the source of the name, were lost in the mist of time." Lost to Moridin, maybe, but not to us!
Steve Deffeyes tells us: "The story of the Fisher King has origins dating back to Celtic times. Originally it is the tale of a king who was stabbed through both thighs, or sometimes is it the side, with a spear thus causing his lands to fall to waste. There's something about early customs forbidding the rule of a blemished king but the true origins of the story are lost, which suits our WOT parallel just fine. Chretien de Troyes introduced it to the Arthurian cycle and everyone from Malory to T.S. Eliot has used it. He has been called King Pelles, Parlan, Bron and Anfortas among others. Sir Balin dealt him the Dolorous Stroke, wounding him with the same spear that stabbed Jesus. He could not ride or hunt due to his never-healing wound and took up fishing. Sir Perceval visited him while on the grail quest and saw the procession of the grail, bleeding lance, candelabra and silver platter but was a new and shy knight and failed to ask the critical question that would have cured the king. Later when Sir Galahad achieved the grail he anointed the king's wounds with the blood of the lance and he was cured."
Here are some additional details from John Johnson:
The Fisher King is the guardian of the Holy Grail. (In some of the earliest legends, this was known as the San Greal.) The Holy Grail was originally brought over from Israel by Joseph of Arimathea, and contained some of Jesus' blood. This explained the magical abilities of the Grail. Legends conflict as to whether his wounding was a result of pride or some other sin. He was directly tied to the land. The land could not be healthy as long as the Fisher King was wounded. He was a powerful magician; some people regarded him as being evil, others good. (Those legends which regard him as being evil cast him as the archetype of Satan.)
Some legends seem to cast the Fisher King as the archetype of mankind, with the wound being the Original Sin, while others show the Fisher King as being symbolic of Christ, with the wound being a representation of his suffering on the cross, or, in some cases, the evil of mankind. This is further confused by the title that Chretien de Troyes gives him. In medieval French it is "Roi des Pecheurs," which can be translated either The King of Fishers, or the King of Sinners, both of which could apply to Jesus Christ.
The quest of the Grail Knights was to seek out the Grail King, and ask him the Grail Question. When they did this, they were then able to heal the King, and thus heal the land. The legends have various knights succeeding to various degrees. Launcelot was able to see it from a distance, both Perceval and Galad were able to answer the Question. Some legends have Bors de Ganis also achieving the Quest with Perceval and Galad, but while they stayed behind, Bors returned to the world to explain what had happened.
Associated with the Arthurian legend of the Fisher King are four objects, the Hallows (there were other Hallows, but these four were most directly involved in the Arthurian legends), that were essential to heal the Maimed King. These were sacred objects, capable of great power. They were:
All of these were directly linked to the Maimed King's sovereignity, and to the idea of power. When Rand goes into beggar-mode, he will lose his power, and these will be necessary to restore him.