This slang expression is used in English meaning mostly “awesome”, “cool”, “excellent”.
Ex. Your shirt is so ace! I love it!

Furthermore, it can be used to refer to a very skilled person, expert, adept.
Ex. My grandfather was a World War I ace pilot.
Andrew is an ace mechanic, he repaired my car in one hour.

It means also “the best”, deriving from aces, the highest cards in poker and other card games.
Ex. That new CD is ace. I’m so glad I bought it, I’m listening to it over and over again.

In addition, you could use “ace” referring to someone who is really close to you, a person you can relate to.
Ex. That’s my ace! She’s my best friend.

As a verb (to ace), it’s used to talk about someone who accomplished something with complete success.
Ex. He’s a champion who could ace it every time.
She aced her entrance exams.