During the first decade of this century, Flash technology was ubiquitous on websites. It was exciting to many, allowing for animation, scripting, interactive applications, and games playing. It could enable your website to do more than just “sit there”. It certainly had its day; I worked in it from time to time.
However, two trends sounded Flash’s death-knell (in my mind): Apple’s decision to not support it on its remote devices (iPhones, iPads) and the proclivity of scammers to leverage flaws in Flash to invade users’ computers.
This short article from Small Dog explains it with perfect conciseness. (Scroll down to “What the heck is Adobe Flash Player?”) It has some especially good advice for you if you still have the Flash plug-in installed on your computer.