A controlling factor in your life, or some vital point. Also, ambiguousness, as the ace can be high or low.
Ace of Hearts: Love and good relationship.
Ace of Diamonds: Financial good fortune.
Ace of Spades: Feeling of success in work.
Ace of Clubs: Wealth, health, love and happiness. A letter concerning money.
Idioms: Ace in the hole (a reserve).
Useful Questions and Hints:
To find out whether the ace is high or low, and what it means in your dream try Talking As.