Snapchat and certain Android phones have used the screen as a flash for years. But this will feel new to iPhone users, and Apple has a installed a special piece of new hardware to make it better.
The problem with LED flash is that it creates a burst of bright light from a single point, and that can create harsh shadows and areas of "blown-out" white space. That's why you often look so horrendous in flash photos, particularly indoors.
Professional photographers get around this by using very large flash devices, or multiple flashes arranged in a circle around the face of a model. That gives you lots of light and no shadows, which is a much kinder way to take pictures. (The picture here shows a really big macro flash, from Canon.)On the new iPhone 6S, when the flash is called for it will activate a new display chip, according to Apple: "Retina Flash is powered by an innovative technology - a custom display chip that allows the display to flash three times brighter than usual."
The flash will thus come from a much larger surface area, creating fewer harsh angles when it hits your face. The camera itself was also improved, getting a megapixel boost, up to 5 from 1.2-megapixels, so that selfies will look more detailed.
The overall result ought to be that selfies taken by iPhone 6S users will now look a lot more beautiful than those taken with LED flash or no flash.
This is not going to be trivial: Snapchat, Facebook, and Instagram are the largest media on the planet, and the selfie (or profile pic) is the default calling card of everyone online. Apple may have come up with a way to make Apple users look better in selfies ... and given that camera quality is half the value to consumers of any given smartphone, this small but clever change could be huge for users.