As Wikipedia notes...
Flash is a multimedia platform originally acquired by Macromedia and currently developed and distributed by Adobe Systems.
Flash has become popular for adding animation and interactivity to web pages. Flash is commonly used to create animation, advertisements, and various web page components, to integrate video into web pages, and to develop rich Internet applications.
Flash is an extremely powerful and sophisticated program. As it can be overwhelming, I'm working on grasping the basics for now!