On television or the radio, a newsflash is an important, timely piece of news that's delivered quickly. If a tornado is headed for your city, your favorite soap opera will be interrupted by a newsflash.
Some news bulletins take the form of a "news crawler" or "ticker," a line of type that scrolls across the bottom of your TV screen. A newsflash is different, because it actually interrupts whatever show you're watching. The information conveyed in a newsflash is seen as being important enough for viewers to take a quick break, so that they don't miss it.