Well if you are referring to the Da Vinci code as in the work of fiction by Dan Brown, then the Holy Grail that he pertains to primarily refers to the 'royal bloodline' i.e. descendants of Jesus, who miraculously, happen to be the hero & heroine of the story. Can you imgaine that?! Sorry if I spoilt the story.
During the times of the crusade, many adventurers believed that the grail was the cup used at the last supper. Often being envisaged as a great solid silver bejewelled chalice, which is highly unlikely for a peasants crockery collection in the middle east in the 1st Century.
Many stories tell of adventurers se\rching for their own grail, which are the version I prefer to believe in, and these are the tails of those who seek true meaning to thier own life - not a physical object. There are thousands of these stories, each single one is better than the Dan Brown nonsense. But to have an opiniohn, you might as well have a read of it and form your own.