Means "congrats!" or "well done!"
Americans say "good for you!" Australians say "good on you!" (spoken: goodonya!) Both of these phrases sound weird to me now.
"Good for you" sounds like a commercial for broccoli or brussel sprouts. It also sounds like something that should be accompanied by a pat on the head. "Good on ya" sounds like a commercial for laundry detergent. (Can you ever get good off ya?)