Adobe's "Flash" technology was once ubiquitous. Ten years ago, if a website had some sort of animation, video or audio on it, chances were that Flash was involved in that process. However, as technology has moved on, Flash has sidelined.
In its later years, it became notorious for the wrong reasons. Despite the fact that usage was decreasing, it was also the case that you could guarantee it would be installed on almost every machine. Therefore it was an appealing target for hackers. It soon got to the point that running it was becoming more trouble than it was worth. Apple started nudging their users away from it by disabling it by default. Others followed, and eventually Adobe conceeded and announced they would be ending all support for the once world-conquering technology.
That end of support date is now within reach. On December 31st, 2020, Adobe will stop creating security updates for the aging tech which, in effect, will mean the end of the technology. Unlike when Microsoft end support for Windows but it carries on working (albeit unsecurely), people rarely run Flash as a standalone application. It's invariably deployed as part of a website, and therefore relies on the web browser to display it to the user.
It has been the case for a while that browsers have made it difficult to run any Flash elements of a website, but if you're determined enough, it has been possible to confirm you really are sure that you want to run it. By the end of the year, this will no longer be possible. Once support is ended at the end of the year, browsers will be removing all options to run the technology, no matter how many warnings you're happy to click through.
For many, they're unlikely to notice any change. With this deadline looming, almost nothing new has been created using Flash for some time. However, it's just worth taking note of any old intranet pages or possibly even full websites you might be using that use the technology for critical purposes. Things like menus and file uploaders often were Flash-based once upon a time. We've also seen a handful of large companies using it for critical parts of their websites, even now. These pages will either need to be rewritten or abandoned by the end of the year.
As a user there will not be much you can do if you encounter a web page that needs Flash to run. Just be alert to the fact that if a webpage you use suddenly stops working in the new year, this is likely to be the cause, and there will be no way to make it work again.