As defined by Joseph Campbell, the grail is the symbol for the achievement of the highest human spiritual potential. Sir Percival became the grail king after realizing the grail's true meaning.
"Only the Grail can redeem the Wasteland, yet what is the Wasteland but the absence of the Grail? Before this, Parzival lived in the Wasteland, but did not suffer it; now he experiences the anguish of that life and so takes on symbolically the wound of the Grail king whose maiming is the expression of the Wasteland. For only when Parzival has healed himself will he be able to heal Anfortas and take upon himself the role of king. But what is the Wasteland? For Campbell it is simply the inauthentic life, a state of being which is barren of the truth of who you are." Joseph Campbell and the Grail Myth - Jules Cashford
http://www.julescashford.com/campbell.html