A blinkie is a round, flat reflector with a little LED blinking light in it. It's not very bright but is just enough to maybe catch someone's attention when it's on the back of your helmet, just in case the rear light on your back is for some reason blocked from their view. It's an el cheapo thing, and while in theory you can keep them going by replacing the little watch battery they run on, ours always seem to die regardless.