What is the star?

I was exactly like you: I asked other users what's the star and when do I receive one. But the answer is written in the FAQ. If we are new users we are not excused for not reading them. The colors of the stars have correspondence to the numbers inside the stars. blue = 10 red = 9 yellow = 8 white = 7 and so on. You'll get your star when you'll have more than 100 posts.