The story of the Fisher King is a story of death and rebirth, not only of the wounded king
himself, but of the land over which he rules. This can be seen even by looking at the name of the myth and its central character: The Fisher King
The Fisher King was indeed called the Fisher King because he fished, but why he fished is the greater mystery. Since time immemorial bodies of water have been potent symbols of death and the unconscious, which were closer things once upon a time than they are now, for in days before photographs, it was only in dreams that we saw the dead live again.
Fish, who live in that water, are often a sign of rebirth, reincarnation, and other means of life from death, for they are the dynamic things that move even in the world under the surface of death. Pisces, of the Zodiac, is a powerful symbol of rebirth, coming as it does at the end of the Zodiac and the turning of winter into spring. Jesus is often given the name Icthus, an acronym in Greek of Jesus of Nazereth, Son of God. Even before the dawn of Christianity, fish were a powerful symbol of rebirth -- making appearances in the Judaism from which Christianity derived as well as other religions. Fishing is, then, the act of bringing about rebirth, of bringing life out of death.
There are many versions of the Fisher King myth, but common elements tend to run through them. The most prominent is, of course, the Fisher King himself, who is always wounded, typically in the groin, or in the side or thigh which are both symbolic of the groin.
The second common element is The Waste Land. The land over which the Fisher King rules is almost invariably infertile. This infertility is because of the King's own infertility -- his health is the health of the land, which is a typical theme in early Indo-European myth and, indeed, belief. There are many cultures that killed the king when their land was failing in the belief that his health was directly tied to the land, there are even some cultures where the king's wife would tell the priests when he was no longer able to satisfy her in bed, as an early warning of the failing of the land -- the king is dead, long live the king.
The third and fourth elements are the lance and the grail, that is, The Holy Grail. These, likewise, are symbols of fertility and are present with the Fisher King in some ritual that is, evidently, designed to heal the king and thus the land. The lance is a potent phallic symbol and the cup unmistakably yonical -- the merging of the two enacts human copulation, another means of restoring fertility.
The stories become divergent upon a third party entering into the story. Often, the person is an innocent who knows nothing of what is going on. Sometimes his innocence saves the king and the land, sometimes it dooms it. Sometimes the innocent even grows to find himself in the same position as the Fisher King towards the end of his life, or sometimes the once-innocent's own King falls victim to the same malady.
Whether the innocent saves or dooms the king, however, the tale remains a potent symbol of death and rebirth, and an allegory, if a thing can be called an allegory when those who created it believed the link to me more than metaphorical, for the turning of the seasons and the emerging of Spring's new life from the ashes of Winter.
King Arthur himself becomes a Fisher King figure at the end of his life, asking one of his knights to cast Excalibur, another potent male symbol, into the lake whence it came, the water here playing the role of a feminine symbol as well as a symbol of death, for afterwards Arthur is himself borne away across the waters to Avalon , the isle of apples - another symbol that recalls the feminine and death at least within Judeo-Christian canon as well as recalling the pomegranate of which Proserpene ate which required her to stay half the year in the land of the dead, to return again, reborn, when he is needed.
The innocent, or The Fool restarts the cycle whether it does so by beginning the king's life anew or beginning the life of an innocent out of the ashes of experience. The Fool is a potent symbol of new beginnings, holding as he does, the zeroth position in the Major Arcana of the Tarot. Indeed, he still has a day at the point where winter becomes Spring in our current calendar -- April Fools Day.