Without any more specific information, this is the best answer I can offer you: "A gold star" can refer to either an actual reward (as mentioned by jura, often given to school children from their teachers in the form of a sticker on their homework or test paper), or it can be a sarcastic remark meant to say "Who cares?" in response to someone saying that they completed or did something well. Again, for a more specific answer, we need to know the context.

Sorry, I have no idea what you are talking about.

Some teachers give their students gold stars as a reward for good work. If someone says, "you deserve a gold star for Oral English", it means that you have done an excellent job.

What are you talking about? Please provide more info.

a star that's made of gold? :P Do I get a gold star? :P