The Ace of Cups symbolises love: the spiritual love that underpins life, and the joy and happiness it brings. Just as fire makes the world, so love gives it value.
The cup is the Holy Grail: the dove and wafer (with its balanced cross) symbolise peace and harmony, and the spiritual basis (virtues of the soul) which underpins the world. Love cannot be seized, but must be freely given and accepted: Cups and water symbolise receptivity.
Reversed: when the cup is emptied or spilled, the love is lost and we have unhappiness, and perhaps violence or destruction. Times have turned against us and we can only accept that life brings problems as well as joy. Perhaps we bring about our own unhappiness by not recognising what life is offering us, by reacting when calm is required.