Flash is a multimedia platform originally developed by a company called Macromedia and bought by Adobe. It allows users to experience interactive content in the browser that HTML was not originally designed to do, like play games and watch videos. Now that HTML specifications (with HTML5) have "caught up," so to speak, with the everyday needs of users (watch videos, play games, etc), Flash is being "turned off" and discontinued in major browsers.
Apple was one of the first major tech company to stop supporting flash. Steve Jobs wrote an infamous article in 2010 titled "Thoughts on Flash," in which he outlines the many flaws with Flash technology. The article was a response to Adobe’s criticism of Apple for removing flash support on Apple devices, such as the iPad and iPhone.
"Flash is a successful business for Adobe, and we can understand why they want to push it beyond PCs. But the mobile era is about low power devices, touch interfaces and open web standards – all areas where Flash falls short…"
Jobs predicted HTML5 standards would "win on mobile devices (and PCs too)," and so far his prediction has been correct.