A quick scan through Wikipedia tells the researcher that the origins of this mystical phrase (that’s ‘Holy Grail’, not ‘Wine Writer’ – though the way ‘I go off on one’ sometimes . . .) are open to considerable conjecture. Also, the phrase has oft been used to signify several different things and is now firmly embedded in the English language.
Its modern use, essentially is to refer to something that is sacred (usually in the non-religious sense), something extremely valuable (not necessarily in monetary terms) something to be searched for, albeit perhaps ultimately unobtainable.