A dragon is a type of coloration on the betta. Usually you can tell what a dragon is because the scales look thicker -as if there's multiple scales piled on top of each other-, metallic scaling on top, and are often times outlined in another coloration. There's nothing wrong with being a dragon, it's actually a coloration that many people like the look of. SOMETIMES, however, the dragon scaling can end up on the bettas' eyes, hampering the ability to see somewhat.
NOW, that was just my rather bad explanation (: I'm sure some of the breeders would be much better at explaining.
Picture's aren't mine.