A flake is a tiny piece of something. A flake could be a tasty morsel like a croissant flake, or a not so tasty morsel — like a dandruff flake. Ewww.
In some places, you can see flakes of mica, a shiny glimmering mineral, lining the banks of a pond or paths through the woods. There are also croissant flakes, dandruff flakes, and of course snow flakes, individual snow crystals. When something flakes, it comes off in tiny flecks: "The paint had started to flake off the kitchen wall." If someone calls you a flake, they basically mean you're a weirdo.