In standard usage, the Holy Grail is a cup or platter used at the last supper by Christ and his disciples. This object has then become the quintessential goal of knightly quests. The Grail is used allegorically to denote the highest achievement of any field, for example, producing a unified theory of the four forces of gravity, electromagnetic, weak and strong nuclear forces could be called the holy grail of physics.
The field of claimed esoteric culture is full of references to the Grail in a bewildering array of contexts and constructions.
The Grail mythos was popularized in the 12th century by Chretien de Troyes. This was followed by a multiplicity of prequels and sequels, most claiming to draw on some hidden tradition or secret source. The only thing that seems relatively sure is that much confusion was spread over a probably ancient, pre-Christian complex of myth.
Secret History analyzes the topic in some detail. The scholars Littleton and Malcor in their book From Scythia to Camelot, a Radical Reassessment of the Legends of King Arthur trace the Grail theme to the Nart sagas of Ossetia, in Southern Caucasus. The themes of the cup, lance etc. would have come with Alanic mercenaries from Iran/Caucasus to the British Isles and Western Europe in the 5th century with the Roman occupation.
These same tribes would then have integrated with the indigenous population and formed a large part of the nobility of the Middle Ages.
Chretien de Troyes was himself sponsored by these families, wherein an oral tradition might have survived.
It appears that the characters of Lancelot, King Arthur and Batros of the Nart sagas are versions of one theme. When the different lines of transmission merged in the Middle Ages, were Christianized and absorbed some of the Celtic lore, many of the characters were alternately split and merged, resulting in much variation on basic themes.
The diverse esoteric overlays on the Grail theme are plausible as such, in their own context but there is no guarantee that they have any relation to the original theme. They could as well be disinformation and a smoke screen, but then even a smoke screen will contain some truth if it is to be effective.