The stars indicate how successful you have been in finishing a level in the campaign. The faster you defeat your opponent, the more stars you receive. The bar in the upper screen area shows the time remaining and the number of stars left for you to get in this level. In the overview map, the number of stars you have been awarded in each level is displayed. Of course, you should always try to get three stars. Nevertheless, you can play every level as often as you want until you get all three stars.