Flash is one of the first pieces of web-based software for running media. Originally Flash was how you got animations and video on the internet.
YouTube dropped Flash as it's video format in 2015.
Flash is notoriously susceptible to virus and malware attacks. In addition, it is a technology that requires it to be continuously running, meaning it takes a lot of processing power from your computer and can slow down your browser's responsiveness. The limitations of Flash, from a power and processing consumption standpoint but also as a security risk, are why Steve Jobs
refused to allow the iPhone and iPad to run Flash.
Adobe, who created and maintains Flash, has announced it will
no longer support Flash as of 2020.
In the meantime there are still a few educational website that require Flash. Sites like Wixie and OneMoreStory. If you are having difficulty running Flash-enabled sites check out this tutorial for solving Flash issues in Chrome...