An ace is a playing card with the highest value in a deck. There are four aces in a full deck of cards, each with a single heart, spade, diamond, or club pictured on it.
Ace can also describe someone who's excellent at a particular skill: "He's an ace at making homemade jam!" In tennis, an ace is a point scored on a missed serve, and in golf it's a hole in one. When you ace something, you do a great job: "Just get in there and ace that math test today!" The earliest meaning was "one at dice," from the Latin as, "a unit, one, or a whole."