That the grail is what people "bring to the table". Much like the Roman saying, Put your best foot foreward. It is much like the grail is what is within each person, what Socrates called his Daimon.

Socrates, I believe, thought it is a gift that everyone possesses, each person's different than the next persons.

It is interesting that this particular grail is associated with great leaders like Jesus and King Arthur. Perhaps by using this metaphor, the ancient authors were comparing King Arthur with Jesus, making him a philosopher king, much like the idealistic leader that Socrates wished for.

But who knows.